diff -r 5ef49659935f Lib/argparse.py
--- a/Lib/argparse.py	Fri Dec 28 19:09:41 2012 +0100
+++ b/Lib/argparse.py	Fri Dec 28 20:12:37 2012 -0500
@@ -1397,7 +1397,11 @@
         # NOTE: if add_mutually_exclusive_group ever gains title= and
         # description= then this code will need to be expanded as above
         for group in container._mutually_exclusive_groups:
-            mutex_group = self.add_mutually_exclusive_group(
+            if group._container is container:
+                cont = self
+            else:
+                cont = title_group_map[group._container.title]
+            mutex_group = cont.add_mutually_exclusive_group(
                 required=group.required)
 
             # map the actions to their new mutex group
diff -r 5ef49659935f Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Fri Dec 28 19:09:41 2012 +0100
+++ b/Lib/test/test_argparse.py	Fri Dec 28 20:12:37 2012 -0500
@@ -2284,6 +2284,36 @@
               -x X
         '''.format(progname, ' ' if progname else '' )))
 
+    def test_mutex_groups_parents(self):
+        parent = ErrorRaisingArgumentParser(add_help=False)
+        g = parent.add_argument_group(title='g', description='gd')
+        g.add_argument('-w')
+        g.add_argument('-x')
+        m = g.add_mutually_exclusive_group()
+        m.add_argument('-y')
+        m.add_argument('-z')
+        parser = ErrorRaisingArgumentParser(parents=[parent])
+
+        self.assertRaises(ArgumentParserError, parser.parse_args,
+            ['-y', 'Y', '-z', 'Z'])
+
+        parser_help = parser.format_help()
+        progname = self.main_program
+        self.assertEqual(parser_help, textwrap.dedent('''\
+            usage: {}{}[-h] [-w W] [-x X] [-y Y | -z Z]
+
+            optional arguments:
+              -h, --help  show this help message and exit
+
+            g:
+              gd
+
+              -w W
+              -x X
+              -y Y
+              -z Z
+        '''.format(progname, ' ' if progname else '' )))
+
 # ==============================
 # Mutually exclusive group tests
 # ==============================
diff -r 5ef49659935f Misc/ACKS
--- a/Misc/ACKS	Fri Dec 28 19:09:41 2012 +0100
+++ b/Misc/ACKS	Fri Dec 28 20:12:37 2012 -0500
@@ -1159,6 +1159,7 @@
 Mark Summerfield
 Reuben Sumner
 Marek Šuppa
+Dougal J. Sutherland
 Hisao Suzuki
 Kalle Svensson
 Andrew Svetlov