diff -u b/Lib/test/test_format.py b/Lib/test/test_format.py
--- b/Lib/test/test_format.py	Mon Jan 20 15:50:44 2014 -0600
+++ b/Lib/test/test_format.py	Mon Jan 20 15:39:21 2014 -0600
@@ -224,9 +224,9 @@
         testboth("%o", -0o42, "-42")
         testboth("%o", float(0o42), "42")
         testformat("%r", "\u0378", "'\\u0378'")  # non printable
-        testformat("%a", "\u0378", "'\\u0378'")  # non printable
+        testboth("%a", "\u0378", "'\\u0378'")  # non printable
         testformat("%r", "\u0374", "'\u0374'")   # printable
-        testformat("%a", "\u0374", "'\\u0374'")  # printable
+        testboth("%a", "\u0374", "'\\u0374'")  # printable
 
         # alternate float formatting
         testboth('%g', 1.1, '1.1')
diff -u b/Objects/bytesobject.c b/Objects/bytesobject.c
--- b/Objects/bytesobject.c	Mon Jan 20 15:50:44 2014 -0600
+++ b/Objects/bytesobject.c	Mon Jan 20 15:39:21 2014 -0600
@@ -587,6 +587,7 @@
             int isnumok;
             PyObject *v = NULL;
             PyObject *temp = NULL;
+            PyObject *temp2;
             Py_buffer buf;
             char *pbuf;
             int sign;
@@ -746,6 +747,19 @@
                 pbuf = "%";
                 len = 1;
                 break;
+            case 'a':
+                temp2 = PyObject_ASCII(v);
+                if (temp2 == NULL)
+                    goto error;
+                temp = PyUnicode_AsLatin1String(temp2);
+                Py_DECREF(temp2);
+                if (temp == NULL)
+                    goto error;
+                pbuf = PyBytes_AS_STRING(temp);
+                len = PyBytes_GET_SIZE(temp);
+                if (prec >= 0 && len > prec)
+                    len = prec;
+                break;
             case 's':
                 if (_getbuffer(v, &buf) < 0) {
                     temp = format_obj(v);