diff -r 9ea84f006892 Lib/argparse.py
--- a/Lib/argparse.py	Wed May 01 15:15:50 2013 +0200
+++ b/Lib/argparse.py	Mon Sep 09 15:27:36 2013 -0700
@@ -1801,12 +1801,15 @@
 
         def take_action(action, argument_strings, option_string=None):
             seen_actions.add(action)
-            argument_values = self._get_values(action, argument_strings)
+            argument_values, using_default = self._get_values(action, argument_strings)
 
             # error if this argument is not allowed with other previously
             # seen arguments, assuming that actions that use the default
             # value don't really count as "present"
-            if argument_values is not action.default:
+            # refine test so that only values set by _get_values() to
+            # action.default count as not-really-present
+
+            if not using_default:
                 seen_non_default_actions.add(action)
                 for conflict_action in action_conflicts.get(action, []):
                     if conflict_action in seen_non_default_actions:
@@ -2212,6 +2215,7 @@
     # ========================
     def _get_values(self, action, arg_strings):
         # for everything but PARSER, REMAINDER args, strip out first '--'
+        using_default = False
         if action.nargs not in [PARSER, REMAINDER]:
             try:
                 arg_strings.remove('--')
@@ -2224,6 +2228,7 @@
                 value = action.const
             else:
                 value = action.default
+                using_default = True
             if isinstance(value, str):
                 value = self._get_value(action, value)
                 self._check_value(action, value)
@@ -2234,6 +2239,7 @@
               not action.option_strings):
             if action.default is not None:
                 value = action.default
+                using_default = True
             else:
                 value = arg_strings
             self._check_value(action, value)
@@ -2260,7 +2266,7 @@
                 self._check_value(action, v)
 
         # return the converted value
-        return value
+        return value, using_default
 
     def _get_value(self, action, arg_string):
         type_func = self._registry_get('type', action.type, action.type)
diff -r 9ea84f006892 Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Wed May 01 15:15:50 2013 +0200
+++ b/Lib/test/test_argparse.py	Mon Sep 09 15:27:36 2013 -0700
@@ -2689,6 +2689,52 @@
           -c          c help
         '''
 
+
+class TestMutuallyExclusiveDefaults(MEMixin, TestCase):
+    '''Narrow seen_non_default_actions criteria:
+    _get_values() sets argument_values = action.default only for positionals
+    with empty strings and '?*' nargs.  Optionals with a value that equals
+    (or 'is') the default don't count
+    '''
+    def get_parser(self, required=None):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        group = parser.add_mutually_exclusive_group(required=required)
+        group.add_argument('--foo', default='test')
+        group.add_argument('--bar', default='test')
+        group.add_argument('--small', type=int, default=42)
+        group.add_argument('--large', type=int, default=257)
+        return parser
+
+    failures = ['--foo X --bar Y',
+                '--small 0 --large 34',
+                '--foo test --bar Y',
+                '--large 257 --foo X',
+                '--small 42 --foo X',
+                ]
+    successes = [
+        ('--foo X', NS(bar='test', foo='X', large=257, small=42)),
+        ('--small 42', NS(bar='test', foo='test', large=257, small=42)),
+    ]
+    successes_when_not_required = [
+        ('', NS(foo='test', bar='test', small=42, large=257)),
+    ]
+    usage_when_required = '''\
+        usage: PROG [-h] (--foo FOO | --bar BAR | --small SMALL | --large LARGE)
+        '''
+    usage_when_not_required = '''\
+        usage: PROG [-h] [--foo FOO | --bar BAR | --small SMALL | --large LARGE]
+        '''
+    help = '''\
+
+        optional arguments:
+          -h, --help     show this help message and exit
+          --foo FOO
+          --bar BAR
+          --small SMALL
+          --large LARGE
+        '''
+
+
 # =================================================
 # Mutually exclusive group in parent parser tests
 # =================================================