diff --git a/ChangeLog b/ChangeLog
index c1de680ef8..512b915c6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -17,6 +17,17 @@ Tue Oct  6 06:26:00 2009  Hidetoshi NAGAI  <nagai@ai.kyutech.ac.jp>
 	* ext/tk/lib/tk/canvas.rb, ext/tk/lib/tkextlib/blt/component.rb: 
 	  lack of support for methodcall_optkeys.
 
+Mon Oct  5 17:19:33 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* lib/delegate.rb (Delegator::public_api): take snapshot of
+	  public method at the beginning time.
+
+	* lib/delegate.rb (SimpleDelegator#initialize): use
+	  Delegator.public_api since public_method might be added after
+	  initialization.  [ruby-dev:39383]
+
+	* lib/delegate.rb (DelegateClass): ditto.
+
 Mon Oct  5 12:22:12 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* array.c (rb_ary_{times, shuffle_bang, sample}): reducing macro
diff --git a/lib/delegate.rb b/lib/delegate.rb
index d0b732aa8d..57c479c61b 100644
--- a/lib/delegate.rb
+++ b/lib/delegate.rb
@@ -196,6 +196,11 @@ class Delegator
     new.__setobj__(__getobj__.dup)
     new
   end
+
+  @delegator_api = self.public_instance_methods
+  def self.public_api   # :nodoc:
+    @delegator_api
+  end
 end
 
 #
@@ -228,6 +233,17 @@ class SimpleDelegator<Delegator
     raise ArgumentError, "cannot delegate to self" if self.equal?(obj)
     @delegate_sd_obj = obj
   end
+
+  def initialize(obj)   # :nodoc:
+    (self.public_methods - Delegator.public_api).each do |m|
+      class << self
+        self
+      end.class_eval do
+        undef_method m
+      end
+    end
+    super
+  end
 end
 
 # :stopdoc:
@@ -257,7 +273,7 @@ end
 def DelegateClass(superclass)
   klass = Class.new(Delegator)
   methods = superclass.public_instance_methods(true)
-  methods -= ::Delegator.public_instance_methods
+  methods -= ::Delegator.public_api
   methods -= [:to_s,:inspect,:=~,:!~,:===]
   klass.module_eval {
     def __getobj__  # :nodoc:
diff --git a/test/test_delegate.rb b/test/test_delegate.rb
index 0b4c117ebd..aecabe147d 100644
--- a/test/test_delegate.rb
+++ b/test/test_delegate.rb
@@ -29,4 +29,26 @@ class TestDelegateClass < Test::Unit::TestCase
     simple=SimpleDelegator.new([])
     assert_equal(SimpleDelegator,simple.class)
   end
+
+  class Object
+    def m
+      :o
+    end
+  end
+
+  class Foo
+    def m
+      :m
+    end
+  end
+
+  class Bar < DelegateClass(Foo)
+  end
+
+  def test_override
+    assert_equal(:o, Object.new.m)
+    assert_equal(:m, Foo.new.m)
+    assert_equal(:m, SimpleDelegator.new(Foo.new).m)
+    assert_equal(:m, Bar.new(Foo.new).m)
+  end
 end