Index: Python/ceval.c
===================================================================
--- Python/ceval.c	(revision 69603)
+++ Python/ceval.c	(working copy)
@@ -1283,7 +1283,10 @@
 		case BINARY_MODULO:
 			w = POP();
 			v = TOP();
-			x = PyNumber_Remainder(v, w);
+			if (PyString_CheckExact(v))
+				x = PyString_Format(v, w);
+			else
+				x = PyNumber_Remainder(v, w);
 			Py_DECREF(v);
 			Py_DECREF(w);
 			SET_TOP(x);
Index: Lib/test/test_opcodes.py
===================================================================
--- Lib/test/test_opcodes.py	(revision 69603)
+++ Lib/test/test_opcodes.py	(working copy)
@@ -102,7 +102,13 @@
         g = eval('lambda a=1: None')
         self.assertNotEquals(f, g)
 
+    def test_modulo_of_string_subclasses(self):
+        class MyString(str):
+            def __mod__(self, value):
+                return 42
+        self.assertEqual(MyString() % 3, 42)
 
+
 def test_main():
     run_unittest(OpcodeTest)