diff -r e2ab56295b56 Lib/ctypes/test/test_bitfields.py
--- a/Lib/ctypes/test/test_bitfields.py	Thu Jul 26 17:16:24 2012 +0300
+++ b/Lib/ctypes/test/test_bitfields.py	Thu Jul 26 20:36:28 2012 +0200
@@ -188,6 +188,29 @@
         self.assertEqual(X.c.offset, sizeof(c_short)*2)
 
 
+    def test_subclassed_bitfields(self):
+        class SubByte(c_ubyte):
+            def __repr__(self):
+                return "SubByte(%d)" % self.value
+
+        class A(Structure):
+            _fields_ = [("lo", c_ubyte, 5),
+                        ("hi", c_ubyte, 3)]
+        class B(Structure):
+            _fields_ = [("lo", SubByte, 5),
+                        ("hi", SubByte, 3)]
+        class TestStruct(Union):
+            _fields_ = [("a", A),
+                        ("b", B),
+                        ("all", c_ubyte)]
+
+        t = TestStruct()
+        for (hi, lo) in ((0, 0), (0, 0x1F), (0x03, 0x00), (0x03, 0x1F)):
+            t.a.lo, t.a.hi = lo, hi
+            self.assertEqual(t.all, lo + (hi << 5))
+            self.assertEqual((t.a.hi, t.a.lo), (hi, lo))
+            self.assertEqual((t.b.hi.value, t.b.lo.value), (hi, lo))
+
     def get_except(self, func, *args, **kw):
         try:
             func(*args, **kw)
diff -r e2ab56295b56 Modules/_ctypes/_ctypes.c
--- a/Modules/_ctypes/_ctypes.c	Thu Jul 26 17:16:24 2012 +0300
+++ b/Modules/_ctypes/_ctypes.c	Thu Jul 26 20:36:28 2012 +0200
@@ -2644,7 +2644,8 @@
 }
 
 PyObject *
-PyCData_FromBaseObj(PyObject *type, PyObject *base, Py_ssize_t index, char *adr)
+PyCData_FromBaseObj(PyObject *type, PyObject *base, Py_ssize_t index,
+		    char *adr, Py_ssize_t size)
 {
     CDataObject *cmem;
     StgDictObject *dict;
@@ -2663,7 +2664,13 @@
     assert(CDataObject_Check(cmem));
 
     cmem->b_length = dict->length;
-    cmem->b_size = dict->size;
+    if (size > 0) {
+      /* bitfields encode the bitsize in the lower byte,
+	 and the bit offset in the upper byte
+      */
+      cmem->b_size = size;
+    } else
+      cmem->b_size = dict->size;
     if (base) { /* use base's buffer */
         assert(CDataObject_Check(base));
         cmem->b_ptr = adr;
@@ -2735,7 +2742,7 @@
     dict = PyType_stgdict(type);
     if (dict && dict->getfunc && !_ctypes_simple_instance(type))
         return dict->getfunc(adr, size);
-    return PyCData_FromBaseObj(type, src, index, adr);
+    return PyCData_FromBaseObj(type, src, index, adr, size);
 }
 
 /*
@@ -4752,7 +4759,7 @@
     assert(stgdict); /* Cannot be NULL fr pointer instances */
     return PyCData_FromBaseObj(stgdict->proto,
                              (PyObject *)self, 0,
-                             *(void **)self->b_ptr);
+			       *(void **)self->b_ptr, 0);
 }
 
 static int
diff -r e2ab56295b56 Modules/_ctypes/callproc.c
--- a/Modules/_ctypes/callproc.c	Thu Jul 26 17:16:24 2012 +0300
+++ b/Modules/_ctypes/callproc.c	Thu Jul 26 20:36:28 2012 +0200
@@ -901,7 +901,7 @@
             Py_DECREF(retval);
         }
     } else
-        retval = PyCData_FromBaseObj(restype, NULL, 0, result);
+      retval = PyCData_FromBaseObj(restype, NULL, 0, result, 0);
 
     if (!checker || !retval)
         return retval;
diff -r e2ab56295b56 Modules/_ctypes/ctypes.h
--- a/Modules/_ctypes/ctypes.h	Thu Jul 26 17:16:24 2012 +0300
+++ b/Modules/_ctypes/ctypes.h	Thu Jul 26 20:36:28 2012 +0200
@@ -354,7 +354,7 @@
 
 extern void _ctypes_add_traceback(char *, char *, int);
 
-extern PyObject *PyCData_FromBaseObj(PyObject *type, PyObject *base, Py_ssize_t index, char *adr);
+extern PyObject *PyCData_FromBaseObj(PyObject *type, PyObject *base, Py_ssize_t index, char *adr, Py_ssize_t size);
 extern char *_ctypes_alloc_format_string(const char *prefix, const char *suffix);
 
 extern int _ctypes_simple_instance(PyObject *obj);