diff --git a/lib/mongo/db.rb b/lib/mongo/db.rb
index ba1279f..1287daa 100644
--- a/lib/mongo/db.rb
+++ b/lib/mongo/db.rb
@@ -15,6 +15,7 @@
 # ++
 
 require 'socket'
+require 'mutex_m'
 require 'mongo/mongo'
 require 'mongo/collection'
 require 'mongo/message'
@@ -61,6 +62,8 @@ module XGen
           @name, @host, @port = db_name, host, port
           @socket = TCPSocket.new(@host, @port)
           @strict = false
+          @semaphore = Object.new
+          @semaphore.extend Mutex_m
         end
 
         # Returns an array of collection names. Each name is of the form
@@ -161,23 +164,26 @@ module XGen
         # Send a Query to +collection_name+ and return a Cursor over the
         # results.
         def query(collection_name, query)
-          # TODO synchronize
-          send_to_db(QueryMessage.new(@name, collection_name, query))
-          return Cursor.new(self, collection_name, query.number_to_return)
+          @semaphore.synchronize {
+            send_to_db(QueryMessage.new(@name, collection_name, query))
+            Cursor.new(self, collection_name, query.number_to_return)
+          }
         end
 
         # Remove the records that match +selector+ from +collection_name+.
         # Normally called by Collection#remove or Collection#clear.
         def remove_from_db(collection_name, selector)
-          # TODO synchronize
-          send_to_db(RemoveMessage.new(@name, collection_name, selector))
+          @semaphore.synchronize {
+            send_to_db(RemoveMessage.new(@name, collection_name, selector))
+          }
         end
 
         # Update records in +collection_name+ that match +selector+ by
         # applying +obj+ as an update. Normally called by Collection#replace.
         def replace_in_db(collection_name, selector, obj)
-          # TODO synchronize
-          send_to_db(UpdateMessage.new(@name, collection_name, selector, obj, false))
+          @semaphore.synchronize {
+            send_to_db(UpdateMessage.new(@name, collection_name, selector, obj, false))
+          }
         end
 
         # Alias for #replace_in_db. Normally called by Collection.modify.
@@ -188,9 +194,10 @@ module XGen
         # called by Collection#repsert.
         def repsert_in_db(collection_name, selector, obj)
           # TODO if PKInjector, inject
-          # TODO synchronize
-          send_to_db(UpdateMessage.new(@name, collection_name, selector, obj, true))
-          obj
+          @semaphore.synchronize {
+            send_to_db(UpdateMessage.new(@name, collection_name, selector, obj, true))
+            obj
+          }
         end
 
         # Return the number of records in +collection_name+ that match
@@ -226,7 +233,6 @@ module XGen
         # :ns :: Namespace; same as +collection_name+.
         def index_information(collection_name)
           sel = {:ns => full_coll_name(collection_name)}
-          # TODO synchronize
           query(SYSTEM_INDEX_COLLECTION, Query.new(sel)).collect { |row|
             h = {:name => row['name']}
             raise "Name of index on return from db was nil. Coll = #{full_coll_name(collection_name)}" unless h[:name]
@@ -251,15 +257,17 @@ module XGen
           field_h = {}
           fields.each { |f| field_h[f] = 1 }
           sel[:key] = field_h
-          # TODO synchronize
-          send_to_db(InsertMessage.new(@name, SYSTEM_INDEX_COLLECTION, sel))
+          @semaphore.synchronize {
+            send_to_db(InsertMessage.new(@name, SYSTEM_INDEX_COLLECTION, sel))
+          }
         end
 
         # Insert +objects+ into +collection_name+. Normally called by
         # Collection#insert.
         def insert_into_db(collection_name, objects)
-          # TODO synchronize
-          objects.each { |o| send_to_db(InsertMessage.new(@name, collection_name, o)) }
+          @semaphore.synchronize {
+            objects.each { |o| send_to_db(InsertMessage.new(@name, collection_name, o)) }
+          }
         end
 
         def send_to_db(message)
@@ -288,7 +296,6 @@ module XGen
             end
           end
 
-          # TODO synchronize
           q = Query.new(selector)
           q.number_to_return = 1
           query(SYSTEM_COMMAND_COLLECTION, q).next_object