diff --git a/activesupport/lib/active_support/core_ext/string/bytesize.rb b/activesupport/lib/active_support/core_ext/string/bytesize.rb
new file mode 100644
index 0000000000000000000000000000000000000000..ed051b921e0b72c3e5d2e3ee0720a95f3f437a9f
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/string/bytesize.rb
@@ -0,0 +1,5 @@
+unless '1.9'.respond_to?(:bytesize)
+  class String
+    alias :bytesize :size
+  end
+end
diff --git a/activesupport/lib/active_support/core_ext/string/interpolation.rb b/activesupport/lib/active_support/core_ext/string/interpolation.rb
index d459c03d394cd6c91b0cfe76ebbf36562f486030..d9159b690aeba735a84651d4e6eaae14523e7921 100644
--- a/activesupport/lib/active_support/core_ext/string/interpolation.rb
+++ b/activesupport/lib/active_support/core_ext/string/interpolation.rb
@@ -6,6 +6,7 @@
 =end
 
 if RUBY_VERSION < '1.9'
+  require 'active_support/core_ext/string/bytesize'
 
   # KeyError is raised by String#% when the string contains a named placeholder
   # that is not contained in the given arguments hash. Ruby 1.9 includes and
@@ -24,8 +25,6 @@ def initialize(message = nil)
   # the meaning of the msgids using "named argument" instead of %s/%d style.
 
   class String
-    # For older ruby versions, such as ruby-1.8.5
-    alias :bytesize :size unless instance_methods.find {|m| m.to_s == 'bytesize'}
     alias :interpolate_without_ruby_19_syntax :% # :nodoc:
 
     INTERPOLATION_PATTERN = Regexp.union(
@@ -90,4 +89,4 @@ def %(args)
       end
     end
   end
-end
\ No newline at end of file
+end
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index a23d3f6fefc3ececffd0a0d7bf5bc38fbaa80222..1005a7e7ad07e6dfd5052e483550cdd9e719ae18 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -345,3 +345,10 @@ def test_string_interpolation_raises_an_argument_error_when_mixing_named_and_unn
     assert_raises(ArgumentError) { "%{name} %f" % [1.0, 2.0] }
   end
 end
+
+class StringBytesizeTest < Test::Unit::TestCase
+  def test_bytesize
+    assert_respond_to 'foo', :bytesize
+    assert_equal 3, 'foo'.bytesize
+  end
+end