diff -r ec4b0145f7e5 Lib/functools.py
--- a/Lib/functools.py	Mon Jun 17 15:45:11 2013 +0200
+++ b/Lib/functools.py	Wed Jun 19 09:08:16 2013 +0100
@@ -376,14 +376,18 @@
         if (needle in bases or not hasattr(needle, '__mro__')
                             or not issubclass(cls, needle)):
             continue   # either present in the __mro__ already or unrelated
-        for index, base in enumerate(mro):
-            if not issubclass(base, needle):
+        for i, base in enumerate(mro):
+            if issubclass(base, needle):
+                index = i + 1
+            elif base in bases:
                 break
-        if base in bases and not issubclass(needle, base):
+        if not issubclass(needle, base):
             # Conflict resolution: put classes present in __mro__ and their
             # subclasses first. See test_mro_conflicts() in test_functools.py
             # for examples.
-            index += 1
+            for index, base in enumerate(mro[i + 1:], i + 1):
+                if not issubclass(base, needle):
+                    break
         mro.insert(index, needle)
     return mro
 
diff -r ec4b0145f7e5 Lib/test/test_functools.py
--- a/Lib/test/test_functools.py	Mon Jun 17 15:45:11 2013 +0200
+++ b/Lib/test/test_functools.py	Wed Jun 19 09:08:16 2013 +0100
@@ -940,6 +940,15 @@
             m = mro(c.ChainMap, haystack)
             self.assertEqual(m, [c.ChainMap, c.MutableMapping, c.Mapping,
                                  c.Sized, c.Iterable, c.Container, object])
+        class C(c.defaultdict):
+            pass
+        c.MutableSequence.register(C)
+        bases = [c.MutableSequence, c.Sequence, c.MutableMapping, c.Mapping]
+        for haystack in permutations(bases):
+            m = mro(C, haystack)
+            self.assertEqual(m, [C, c.defaultdict, c.MutableSequence, c.Sequence,
+                                 dict, c.MutableMapping, c.Mapping, object])
+
         # Note: The MRO order below depends on haystack ordering.
         m = mro(c.defaultdict, [c.Sized, c.Container, str])
         self.assertEqual(m, [c.defaultdict, dict, c.Container, c.Sized, object])