Index: Lib/ctypes/test/test_bitfields.py
===================================================================
--- Lib/ctypes/test/test_bitfields.py	(revision 85202)
+++ Lib/ctypes/test/test_bitfields.py	(working copy)
@@ -240,5 +240,25 @@
             _anonymous_ = ["_"]
             _fields_ = [("_", X)]
 
+    @unittest.skipUnless(hasattr(ctypes, "c_uint32"), "c_int32 is required")
+    def test_uint32(self):
+        class X(Structure):
+            _fields_ = [("a", c_uint32, 32)]
+        x = X()
+        x.a = 10
+        self.assertEquals(x.a, 10)
+        x.a = 0xFDCBA987
+        self.assertEquals(x.a, 0xFDCBA987)
+
+    @unittest.skipUnless(hasattr(ctypes, "c_uint64"), "c_int64 is required")
+    def test_uint64(self):
+        class X(Structure):
+            _fields_ = [("a", c_uint64, 64)]
+        x = X()
+        x.a = 10
+        self.assertEquals(x.a, 10)
+        x.a = 0xFEDCBA9876543211
+        self.assertEquals(x.a, 0xFEDCBA9876543211)
+
 if __name__ == "__main__":
     unittest.main()
Index: Modules/_ctypes/cfield.c
===================================================================
--- Modules/_ctypes/cfield.c	(revision 85202)
+++ Modules/_ctypes/cfield.c	(working copy)
@@ -428,13 +428,15 @@
 #define LOW_BIT(x)  ((x) & 0xFFFF)
 #define NUM_BITS(x) ((x) >> 16)
 
-/* This seems nore a compiler issue than a Windows/non-Windows one */
-#ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
+#ifdef HAVE_LONG_LONG
+#  define MAX_SIZE_INT(n) Py_ULL(n)
 #else
-#  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
+#  define MAX_SIZE_INT(n) n##UL
 #endif
 
+/* Doesn't work if NUM_BITS(size) == 0, but it never happens in SET() call. */
+#define BIT_MASK(size) ((((MAX_SIZE_INT(1) << (NUM_BITS(size) - 1)) - 1) << 1) + 1)
+
 /* This macro CHANGES the first parameter IN PLACE. For proper sign handling,
    we must first shift left, then right.
 */