diff -r a858cde113f2 -r 413736d50da1 Lib/ctypes/test/test_bitfields.py
--- a/Lib/ctypes/test/test_bitfields.py	Sat Jan 24 19:26:18 2015 -0800
+++ b/Lib/ctypes/test/test_bitfields.py	Wed Jun 24 17:34:18 2015 +0200
@@ -259,5 +259,18 @@
         x.a = 0xFEDCBA9876543211
         self.assertEqual(x.a, 0xFEDCBA9876543211)
 
+    @need_symbol('c_uint32')
+    def test_uint32_swap(self):
+        class X(BigEndianStructure):
+            _fields_ = [("pad", c_uint32, 16),
+                        ("a"  , c_uint32, 4),
+                        ("b"  , c_uint32, 12)]
+        b = bytearray(4)
+        x = X.from_buffer(b)
+        x.a = 1
+        self.assertEqual(b, b'\x00\x00\x10\x00')
+        x.b = 0x234
+        self.assertEqual(b, b'\x00\x00\x12\x34')
+
 if __name__ == "__main__":
     unittest.main()
diff -r a858cde113f2 -r 413736d50da1 Modules/_ctypes/cfield.c
--- a/Modules/_ctypes/cfield.c	Sat Jan 24 19:26:18 2015 -0800
+++ b/Modules/_ctypes/cfield.c	Wed Jun 24 17:34:18 2015 +0200
@@ -765,6 +765,7 @@
     if (get_ulong(value, &val) < 0)
         return  NULL;
     memcpy(&field, ptr, sizeof(field));
+    field = SWAP_INT(field);
     field = SET(unsigned int, field, (unsigned int)val, size);
     field = SWAP_INT(field);
     memcpy(ptr, &field, sizeof(field));