diff --git a/README.rdoc b/README.rdoc
index aee5944..d4c6985 100644
--- a/README.rdoc
+++ b/README.rdoc
@@ -25,6 +25,10 @@ A quick code sample:
   puts "There are #{coll.count()} records. Here they are:"
   coll.find().each { |doc| puts doc.inspect }
 
+This driver also includes an implementation of a GridStore class, a Ruby
+interface to Mongo's GridFS storage. NOTE: the GridStore code may be moved to
+a separate project.
+
 = Installation
 
 Install the "mongo" gem by typing
@@ -54,6 +58,15 @@ be running, of course.
 
 See also the test code, especially tests/test_db_api.rb.
 
+For the GridFS class GridStore, see the tests.
+
+
+= GridStore
+
+The GridStore class is a Ruby implementation of Mongo's GridFS file storage
+system. An instance of GridStore is like an IO object. See the rdocs for
+details.
+
 
 = Notes
 
diff --git a/lib/mongo/gridfs.rb b/lib/mongo/gridfs.rb
new file mode 100644
index 0000000..d6fc1b2
--- /dev/null
+++ b/lib/mongo/gridfs.rb
@@ -0,0 +1 @@
+require 'mongo/gridfs/grid_store'
diff --git a/lib/mongo/gridfs/chunk.rb b/lib/mongo/gridfs/chunk.rb
new file mode 100644
index 0000000..a500dfd
--- /dev/null
+++ b/lib/mongo/gridfs/chunk.rb
@@ -0,0 +1,107 @@
+require 'mongo/types/binary'
+require 'mongo/types/dbref'
+require 'mongo/types/objectid'
+require 'mongo/util/byte_buffer'
+require 'mongo/util/ordered_hash'
+
+
+module XGen
+  module Mongo
+    module GridFS
+
+      # A chunk stores a portion of GridStore data.
+      #
+      # TODO: user-defined chunk size
+      class Chunk
+
+        DEFAULT_CHUNK_SIZE = 1024 * 256
+
+        attr_reader :object_id, :chunk_number
+        attr_accessor :data
+
+        def initialize(db_collection, mongo_object={})
+          @coll = db_collection
+          @object_id = mongo_object['_id'] || XGen::Mongo::Driver::ObjectID.new
+          @chunk_number = mongo_object['cn'] || 1
+
+          @data = ByteBuffer.new
+          case mongo_object['data']
+          when String
+            mongo_object['data'].each_byte { |b| @data.put(b) }
+          when ByteBuffer
+            @data.put_array(mongo_object['data'].to_a)
+          when Array
+            @data.put_array(mongo_object['data'])
+          end
+          @data.rewind
+
+          @next_chunk_dbref = mongo_object['next']
+        end
+
+        def has_next?
+          @next_chunk_dbref
+        end
+
+        def next
+          return @next_chunk if @next_chunk
+          return nil unless @next_chunk_dbref
+          row = @coll.find({'_id' => @next_chunk_dbref.object_id}).next_object
+          @next_chunk = self.class.new(@coll, row) if row
+          @next_chunk
+        end
+
+        def next=(chunk)
+          @next_chunk = chunk
+          @next_chunk_dbref = XGen::Mongo::Driver::DBRef.new(nil, nil, @coll.db, '_chunks', chunk.object_id)
+        end
+
+        def pos; @data.position; end
+        def pos=(pos); @data.position = pos; end
+        def eof?; !@data.more?; end
+
+        def size; @data.size; end
+        alias_method :length, :size
+
+        def empty?
+          @data.length == 0
+        end
+
+        def clear
+          @data.clear
+        end
+
+        # Erase all data after current position.
+        def truncate
+          if @data.position < @data.length
+            curr_data = @data
+            @data = ByteBuffer.new
+            @data.put_array(curr_data.to_a[0...curr_data.position])
+          end
+        end
+
+        def getc
+          @data.more? ? @data.get : nil
+        end
+
+        def putc(byte)
+          @data.put(byte)
+        end
+
+        def save
+          @coll.remove({'_id' => @object_id}) if @object_id
+          @coll.insert(to_mongo_object)
+        end
+
+        def to_mongo_object
+          h = OrderedHash.new
+          h['_id'] = @object_id
+          h['cn'] = @chunk_number
+          h['data'] = data
+          h['next'] = @next_chunk_dbref
+          h
+        end
+
+      end
+    end
+  end
+end
diff --git a/lib/mongo/gridfs/grid_store.rb b/lib/mongo/gridfs/grid_store.rb
new file mode 100644
index 0000000..a76f2ce
--- /dev/null
+++ b/lib/mongo/gridfs/grid_store.rb
@@ -0,0 +1,397 @@
+require 'mongo/types/dbref'
+require 'mongo/types/objectid'
+require 'mongo/util/ordered_hash'
+require 'mongo/gridfs/chunk'
+
+module XGen
+  module Mongo
+    module GridFS
+
+      # GridStore is an IO-like object that provides input and output for
+      # streams of data to Mongo. See Mongo's documentation about GridFS for
+      # storage implementation details.
+      #
+      # Example code:
+      #
+      #   GridStore.open(database, 'filename', 'w') { |f|
+      #     f.puts "Hello, world!"
+      #   }
+      #   GridStore.open(database, 'filename, 'r') { |f|
+      #     puts f.read         # => Hello, world!\n
+      #   }
+      #   GridStore.open(database, 'filename', 'w+") { |f|
+      #     f.puts "But wait, there's more!"
+      #   }
+      #   GridStore.open(database, 'filename, 'r') { |f|
+      #     puts f.read         # => Hello, world!\nBut wait, there's more!\n
+      #   }
+      class GridStore
+
+        include Enumerable
+
+        attr_accessor :filename
+
+        # Array of strings; may be +nil+
+        attr_accessor :aliases
+
+        # Default is 'text/plain'
+        attr_accessor :content_type
+
+        attr_reader :object_id, :upload_date
+
+        attr_reader :chunk_size
+
+        attr_accessor :lineno
+
+        class << self
+
+          def exist?(db, name)
+            db.collection('_files').find({'filename' => name}).next_object.nil?
+          end
+
+          def open(db, name, mode)
+            gs = self.new(db, name, mode)
+            result = nil
+            begin
+              result = yield gs if block_given?
+            ensure
+              gs.close
+            end
+            result
+          end
+
+          def read(db, name, length=nil, offset=nil)
+            GridStore.open(db, name, 'r') { |gs|
+              gs.seek(offset) if offset
+              gs.read(length)
+            }
+          end
+
+          def readlines(db, name, separator=$/)
+            GridStore.open(db, name, 'r') { |gs|
+              gs.readlines(separator)
+            }
+          end
+
+          def unlink(db, *names)
+            names.each { |name|
+              gs = GridStore.new(db, name)
+              gs.send(:delete_chunks)
+              db.collection('_files').remove('_id' => gs.object_id)
+            }
+          end
+          alias_method :delete, :unlink
+
+        end
+
+        #---
+        # ================================================================
+        #+++
+
+        # Mode may only be 'r', 'w', or 'w+'.
+        def initialize(db, name, mode='r')
+          @db, @filename, @mode = db, name, mode
+
+          doc = @db.collection('_files').find({'filename' => @filename}).next_object
+          if doc
+            @object_id = doc['_id']
+            @content_type = doc['contentType']
+            @chunk_size = doc['chunkSize']
+            @upload_date = doc['uploadDate']
+            @aliases = doc['aliases']
+            @length = doc['length']
+            fc_id = doc['next']
+            if fc_id
+              coll = @db.collection('_chunks')
+              row = coll.find({'_id' => fc_id.object_id}).next_object
+              @first_chunk = row ? Chunk.new(coll, row) : nil
+            else
+              @first_chunk = nil
+            end
+          else
+            @upload_date = Time.new
+            @chunk_size = Chunk::DEFAULT_CHUNK_SIZE
+            @content_type = 'text/plain'
+            @length = 0
+          end
+
+          case mode
+          when 'r'
+            @curr_chunk = @first_chunk
+          when 'w'
+            delete_chunks
+            @first_chunk = @curr_chunk = nil
+          when 'w+'
+            @curr_chunk = find_last_chunk
+            @curr_chunk.pos = @curr_chunk.data.length if @curr_chunk
+          end
+
+          @lineno = 0
+          @pushback_byte = nil
+        end
+
+        # Change chunk size. Can only change if the file is opened for write
+        # and the first chunk's size is zero.
+        def chunk_size=(size)
+          unless @mode[0] == ?w && @first_chunk == nil
+            raise "error: can only change chunk size if open for write and no data written."
+          end
+          @chunk_size = size
+        end
+
+        #---
+        # ================ reading ================
+        #+++
+
+        def getc
+          if @pushback_byte
+            byte = @pushback_byte
+            @pushback_byte = nil
+            byte
+          elsif eof?
+            nil
+          else
+            if @curr_chunk.eof?
+              @curr_chunk = @curr_chunk.next
+            end
+            @curr_chunk.getc
+          end
+        end
+
+        def gets(separator=$/)
+          str = ''
+          byte = getc
+          return nil if byte == nil # EOF
+          while byte != nil
+            s = byte.chr
+            str << s
+            break if s == separator
+            byte = getc
+          end
+          @lineno += 1
+          str
+        end
+
+        def read(len=nil, buf=nil)
+          buf ||= ''
+          byte = getc
+          while byte != nil && (len == nil || len > 0)
+            buf << byte.chr
+            len -= 1 if len
+            byte = getc if (len == nil || len > 0)
+          end
+          buf
+        end
+
+        def readchar
+          byte = getc
+          raise EOFError.new if byte == nil
+          byte
+        end
+
+        def readline(separator=$/)
+          line = gets
+          raise EOFError.new if line == nil
+          line
+        end
+
+        def readlines(separator=$/)
+          read.split(separator).collect { |line| "#{line}#{separator}" }
+        end
+
+        def each
+          line = gets
+          while line
+            yield line
+            line = gets
+          end
+        end
+        alias_method :each_line, :each
+
+        def each_byte
+          byte = getc
+          while byte
+            yield byte
+            byte = getc
+          end
+        end
+
+        def ungetc(byte)
+          @pushback_byte = byte
+        end
+
+        #---
+        # ================ writing ================
+        #+++
+
+        def putc(byte)
+          chunks = @db.collection('_chunks')
+          if @curr_chunk == nil
+            @first_chunk = @curr_chunk = Chunk.new(chunks, 'cn' => 1)
+          elsif @curr_chunk.pos == @chunk_size
+            prev_chunk = @curr_chunk
+            @curr_chunk = Chunk.new(chunks, 'cn' => prev_chunk.chunk_number + 1)
+            prev_chunk.next = @curr_chunk
+            prev_chunk.save
+          end
+          @curr_chunk.putc(byte)
+        end
+
+        def print(*objs)
+          objs = [$_] if objs == nil || objs.empty?
+          objs.each { |obj|
+            str = obj.to_s
+            str.each_byte { |byte| putc(byte) }
+          }
+          nil
+        end
+
+        def puts(*objs)
+          if objs == nil || objs.empty?
+            putc(10)
+          else
+            print(*objs.collect{ |obj|
+                    str = obj.to_s
+                    str << "\n" unless str =~ /\n$/
+                    str
+                  })
+          end
+          nil
+        end
+
+        def <<(obj)
+          write(obj.to_s)
+        end
+
+        # Writes +string+ as bytes and returns the number of bytes written.
+        def write(string)
+          raise "#@filename not opened for write" unless @mode[0] == ?w
+          count = 0
+          string.each_byte { |byte|
+            putc byte
+            count += 1
+          }
+          count
+        end
+
+        # A no-op.
+        def flush
+        end
+
+        #---
+        # ================ status ================
+        #+++
+
+        def eof
+          raise IOError.new("stream not open for reading") unless @mode[0] == ?r
+          @curr_chunk == nil || (@curr_chunk.eof? && !@curr_chunk.has_next?)
+        end
+        alias_method :eof?, :eof
+
+        #---
+        # ================ positioning ================
+        #+++
+
+        def rewind
+          if @curr_chunk != @first_chunk
+            @curr_chunk.save unless @curr_chunk == nil || @curr_chunk.empty?
+            @curr_chunk == @first_chunk
+          end
+          @curr_chunk.pos = 0
+          @lineno = 0
+          # TODO if writing, delete all other chunks on first write
+        end
+
+        def seek(pos, whence=IO::SEEK_SET)
+#           target_pos = case whence
+#                        when IO::SEEK_CUR
+#                          tell + pos
+#                        when IO::SEEK_END
+                         
+#           @curr_chunk.save if @curr_chunk
+#           target_chunk_num = ((pos / @chunk_size) + 1).to_i
+#           target_chunk_pos = pos % @chunk_size
+#           if @curr_chunk == nil || @curr_chunk.chunk_number != target_chunk_num
+#           end
+#           @curr_chunk.pos = target_chunk_pos
+#           0
+          # TODO
+          raise "not yet implemented"
+        end
+
+        def tell
+          return 0 unless @curr_chunk
+          @chunk_size * (@curr_chunk.chunk_number - 1) + @curr_chunk.pos
+        end
+
+        #---
+        # ================ closing ================
+        #+++
+
+        def close
+          if @mode[0] == ?w
+            if @curr_chunk
+              @curr_chunk.truncate
+              @curr_chunk.save
+            end
+            files = @db.collection('_files')
+            if @object_id
+              files.remove('_id' => @object_id)
+            else
+              @object_id = XGen::Mongo::Driver::ObjectID.new
+            end
+            files.insert(to_mongo_object)
+          end
+          @db = nil
+        end
+          
+        def closed?
+          @db == nil
+        end
+
+        #---
+        # ================ protected ================
+        #+++
+
+        protected
+
+        def to_mongo_object
+          h = OrderedHash.new
+          h['_id'] = @object_id
+          h['filename'] = @filename
+          h['contentType'] = @content_type
+          h['length'] = @curr_chunk ? (@curr_chunk.chunk_number - 1) * @chunk_size + @curr_chunk.pos : 0
+          h['chunkSize'] = @chunk_size
+          h['uploadDate'] = @upload_date
+          h['aliases'] = @aliases
+          h['next'] = XGen::Mongo::Driver::DBRef.new(nil, nil, @db, '_chunks', @first_chunk.object_id) if @first_chunk
+          h
+        end
+
+        def find_last_chunk
+          chunk = @curr_chunk || @first_chunk
+          while chunk.has_next?
+            chunk = chunk.next
+          end
+          chunk
+        end
+
+        def save_chunk(chunk)
+          chunks = @db.collection('_chunks')
+        end
+
+        def delete_chunks
+          chunk = @first_chunk
+          coll = @db.collection('_chunks')
+          while chunk
+            next_chunk = chunk.next
+            coll.remove({'_id' => chunk.object_id})
+            chunk = next_chunk
+          end
+          @first_chunk = @curr_chunk = nil
+        end
+
+      end
+    end
+  end
+end
diff --git a/mongo-ruby-driver.gemspec b/mongo-ruby-driver.gemspec
index cab37c8..5b4dc34 100644
--- a/mongo-ruby-driver.gemspec
+++ b/mongo-ruby-driver.gemspec
@@ -1,6 +1,6 @@
 Gem::Specification.new do |s|
   s.name = 'mongo'
-  s.version = '0.3.3'
+  s.version = '0.4.0'
   s.platform = Gem::Platform::RUBY
   s.summary = 'Simple pure-Ruby driver for the 10gen Mongo DB'
   s.description = 'A pure-Ruby driver for the 10gen Mongo DB. For more information about Mongo, see http://www.mongodb.org.'
diff --git a/tests/test_chunk.rb b/tests/test_chunk.rb
new file mode 100644
index 0000000..f1d8762
--- /dev/null
+++ b/tests/test_chunk.rb
@@ -0,0 +1,108 @@
+$LOAD_PATH[0,0] = File.join(File.dirname(__FILE__), '..', 'lib')
+require 'test/unit'
+require 'rubygems'
+require 'mongo'
+require 'mongo/gridfs'
+
+class ChunkTest < Test::Unit::TestCase
+
+  include XGen::Mongo::Driver
+  include XGen::Mongo::GridFS
+
+  def setup
+    @host = ENV['MONGO_RUBY_DRIVER_HOST'] || 'localhost'
+    @port = ENV['MONGO_RUBY_DRIVER_PORT'] || Mongo::DEFAULT_PORT
+    @db = Mongo.new(@host, @port).db('ruby-mongo-utils-test')
+
+    @files = @db.collection('_files')
+    @chunks = @db.collection('_chunks')
+    @chunks.clear
+    @files.clear
+
+    @c = Chunk.new(@chunks)
+  end
+
+  def teardown
+    if @db && @db.connected?
+      @chunks.clear
+      @files.clear
+      @db.close
+    end
+  end
+
+  def test_has_next
+    assert !@c.has_next?
+    @c.next = Chunk.new(@chunks)
+    assert @c.has_next?
+  end
+
+  def test_assign_next
+    assert !@c.has_next?
+    assert_nil @c.next
+
+    c2 = Chunk.new(@chunks)
+    @c.next = c2
+    assert_same c2, @c.next
+  end
+
+  def test_pos
+    assert_equal 0, @c.pos
+    assert @c.eof?              # since data is empty
+
+    b = ByteBuffer.new
+    3.times { |i| b.put(i) }
+    c = Chunk.new(@db, 'data' => b)
+    assert !c.eof?
+  end
+
+  def test_getc
+    b = ByteBuffer.new
+    3.times { |i| b.put(i) }
+    c = Chunk.new(@chunks, 'data' => b)
+
+    assert !c.eof?
+    assert_equal 0, c.getc
+    assert !c.eof?
+    assert_equal 1, c.getc
+    assert !c.eof?
+    assert_equal 2, c.getc
+    assert c.eof?
+  end
+
+  def test_putc
+    3.times { |i| @c.putc(i) }
+    @c.pos = 0
+
+    assert !@c.eof?
+    assert_equal 0, @c.getc
+    assert !@c.eof?
+    assert_equal 1, @c.getc
+    assert !@c.eof?
+    assert_equal 2, @c.getc
+    assert @c.eof?
+  end
+
+  def test_empty
+    assert @c.empty?
+    @c.putc(1)
+    assert !@c.empty?
+  end
+
+  def test_truncate
+    10.times { |i| @c.putc(i) }
+    assert_equal 10, @c.size
+    @c.pos = 3
+    @c.truncate
+    assert_equal 3, @c.size
+
+    @c.pos = 0
+    assert !@c.eof?
+    assert_equal 0, @c.getc
+    assert !@c.eof?
+    assert_equal 1, @c.getc
+    assert !@c.eof?
+    assert_equal 2, @c.getc
+    assert @c.eof?
+  end
+
+end
diff --git a/tests/test_grid_store.rb b/tests/test_grid_store.rb
new file mode 100644
index 0000000..5f19cc9
--- /dev/null
+++ b/tests/test_grid_store.rb
@@ -0,0 +1,177 @@
+$LOAD_PATH[0,0] = File.join(File.dirname(__FILE__), '..', 'lib')
+require 'test/unit'
+require 'rubygems'
+require 'mongo'
+require 'mongo/gridfs'
+
+class GridStoreTest < Test::Unit::TestCase
+
+  include XGen::Mongo::Driver
+  include XGen::Mongo::GridFS
+
+  def setup
+    @host = ENV['MONGO_RUBY_DRIVER_HOST'] || 'localhost'
+    @port = ENV['MONGO_RUBY_DRIVER_PORT'] || Mongo::DEFAULT_PORT
+    @db = Mongo.new(@host, @port).db('ruby-mongo-utils-test')
+
+    @files = @db.collection('_files')
+    @chunks = @db.collection('_chunks')
+    @chunks.clear
+    @files.clear
+
+    GridStore.open(@db, 'foobar', 'w') { |f| f.write("hello, world!") }
+  end
+
+  def teardown
+    if @db && @db.connected?
+      @chunks.clear
+      @files.clear
+      @db.close
+    end
+  end
+
+  def test_small_write
+    rows = @files.find({'filename' => 'foobar'}).to_a
+    assert_not_nil rows
+    assert_equal 1, rows.length
+    assert_equal 1, rows.length
+    row = rows[0]
+    assert_not_nil row
+
+    assert_kind_of DBRef, row['next']
+    first_chunk_id = row['next'].object_id
+    first_chunk = @chunks.find({'_id' => first_chunk_id}).next_object
+  end
+
+  def test_small_file
+    rows = @files.find({'filename' => 'foobar'}).to_a
+    assert_not_nil rows
+    assert_equal 1, rows.length
+    row = rows[0]
+    assert_not_nil row
+    assert_equal "hello, world!", GridStore.read(@db, 'foobar')
+  end
+
+  def test_overwrite
+    GridStore.open(@db, 'foobar', 'w') { |f| f.write("overwrite") }
+    assert_equal "overwrite", GridStore.read(@db, 'foobar')
+  end
+
+  def test_read_length
+    assert_equal "hello", GridStore.read(@db, 'foobar', 5)
+  end
+
+# TODO seek not yet implemented
+#   def test_read_with_offset
+#     assert_equal "world", GridStore.read(@db, 'foobar', 5, 7)
+#     assert_equal "world!", GridStore.read(@db, 'foobar', nil, 7)
+#   end
+
+  def test_multi_chunk
+    @chunks.clear
+    @files.clear
+
+    size = 512
+    GridStore.open(@db, 'biggie', 'w') { |f|
+      f.chunk_size = size
+      f.write('x' * size)
+      f.write('y' * size)
+      f.write('z' * size)
+    }
+
+    assert_equal 3, @chunks.count
+    assert_equal ('x' * size) + ('y' * size) + ('z' * size), GridStore.read(@db, 'biggie')
+  end
+
+  def test_puts_and_readlines
+    GridStore.open(@db, 'multiline', 'w') { |f|
+      f.puts "line one"
+      f.puts "line two\n"
+      f.puts "line three"
+    }
+
+    lines = GridStore.readlines(@db, 'multiline')
+    assert_equal ["line one\n", "line two\n", "line three\n"], lines
+  end
+
+  def test_unlink
+    assert_equal 1, @files.count
+    assert_equal 1, @chunks.count
+    GridStore.unlink(@db, 'foobar')
+    assert_equal 0, @files.count
+    assert_equal 0, @chunks.count
+  end
+
+  def test_append
+    GridStore.open(@db, 'foobar', 'w+') { |f| f.write(" how are you?") }
+    assert_equal 1, @chunks.count
+    assert_equal "hello, world! how are you?", GridStore.read(@db, 'foobar')
+  end
+
+  def test_rewind_and_truncate_on_write
+    GridStore.open(@db, 'foobar', 'w') { |f|
+      f.write("some text is inserted here")
+      f.rewind
+      f.write("abc")
+    }
+    assert_equal "abc", GridStore.read(@db, 'foobar')
+  end
+
+  def test_tell
+    GridStore.open(@db, 'foobar', 'r') { |f|
+      f.read(5)
+      assert_equal 5, f.tell
+    }
+  end
+
+  def test_empty_block_ok
+    GridStore.open(@db, 'empty', 'w')
+  end
+
+  def test_save_empty_file
+    @chunks.clear
+    @files.clear
+    GridStore.open(@db, 'empty', 'w')
+    assert_equal 1, @files.count
+    assert_equal 0, @chunks.count
+  end
+
+  def test_empty_file_eof
+    GridStore.open(@db, 'empty', 'w')
+    GridStore.open(@db, 'empty', 'r') { |f|
+      assert f.eof?
+    }
+  end
+
+  def test_cannot_change_chunk_size_on_read
+    begin
+      GridStore.open(@db, 'foobar', 'r') { |f| f.chunk_size = 42 }
+      fail "should have seen error"
+    rescue => ex
+      assert_match /error: can only change chunk size/, ex.to_s
+    end
+  end
+
+  def test_cannot_change_chunk_size_after_data_written
+    begin
+      GridStore.open(@db, 'foobar', 'w') { |f|
+        f.write("some text")
+        f.chunk_size = 42
+      }
+      fail "should have seen error"
+    rescue => ex
+      assert_match /error: can only change chunk size/, ex.to_s
+    end
+  end
+
+  def test_change_chunk_size
+    GridStore.open(@db, 'new-file', 'w') { |f|
+      f.chunk_size = 42
+      f.write("foo")
+    }
+    GridStore.open(@db, 'new-file', 'r') { |f|
+      assert f.chunk_size == 42
+    }
+  end
+
+end