diff -r 52744a5a9260 Lib/ctypes/test/test_bitfields.py
--- a/Lib/ctypes/test/test_bitfields.py	Fri Jun 24 09:37:26 2011 -0500
+++ b/Lib/ctypes/test/test_bitfields.py	Fri Jun 24 15:29:42 2011 -0700
@@ -63,6 +63,11 @@
         x.a, x.b, x.c = -1, 7, -1
         self.assertEqual((x.a, x.b, x.c), (-1, 7, -1))
 
+        # set all bits in x.b to 1
+        x.a, x.b, x.c = 0, -1, 0
+        self.assertEqual((x.a, x.b, x.c), (0, -1, 0))
+
+
     def test_ulonglong(self):
         class X(Structure):
             _fields_ = [("a", c_ulonglong, 1),
@@ -75,6 +80,11 @@
         x.a, x.b, x.c = 7, 7, 7
         self.assertEqual((x.a, x.b, x.c), (1, 7, 1))
 
+        # set all bits in x.b to 1
+        x.a, x.b, x.c = 0, 0x3fffffffffffffff, 0
+        self.assertEqual((x.a, x.b, x.c), (0, 0x3fffffffffffffff, 0))
+
+
     def test_signed(self):
         for c_typ in signed_int_types:
             class X(Structure):