diff --git a/Lib/pickle.py b/Lib/pickle.py
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -352,7 +352,11 @@
             self.save(pid, save_persistent_id=False)
             self.write(BINPERSID)
         else:
-            self.write(PERSID + str(pid).encode("ascii") + b'\n')
+            try:
+                self.write(PERSID + str(pid).encode("ascii") + b'\n')
+            except UnicodeEncodeError:
+                raise PicklingError(
+                    "persistent IDs in protocol 0 must be ASCII strings")
 
     def save_reduce(self, func, args, state=None,
                     listitems=None, dictitems=None, obj=None):
@@ -870,7 +874,11 @@
     dispatch[PROTO[0]] = load_proto
 
     def load_persid(self):
-        pid = self.readline()[:-1].decode("ascii")
+        try:
+            pid = self.readline()[:-1].decode("ascii")
+        except UnicodeDecodeError:
+            raise UnpicklingError(
+                "persistent IDs in protocol 0 must be ASCII strings")
         self.append(self.persistent_load(pid))
     dispatch[PERSID[0]] = load_persid
 
diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -1467,6 +1467,35 @@
         self.assertEqual(self.load_count, 5)
 
 
+class AbstractIdentityPersistentPicklerTests(unittest.TestCase):
+
+    def persistent_id(self, obj):
+        return obj
+
+    def persistent_load(self, pid):
+        return pid
+
+    def _check_return_correct_type(self, obj, proto):
+        unpickled = self.loads(self.dumps(obj, proto))
+        self.assertIsInstance(unpickled, type(obj))
+        self.assertEqual(unpickled, obj)
+
+    def test_return_correct_type(self):
+        for proto in protocols:
+            # Protocol 0 supports only ASCII strings.
+            if proto == 0:
+                self._check_return_correct_type("abc", 0)
+            else:
+                for obj in [b"abc\n", "abc\n", -1, -1.1 * 0.1, str]:
+                    self._check_return_correct_type(obj, proto)
+
+    def test_protocol0_is_ascii_only(self):
+        non_ascii_str = "\N{EMPTY SET}"
+        self.assertRaises(pickle.PicklingError, self.dumps, non_ascii_str, 0)
+        pickled = pickle.PERSID + non_ascii_str.encode('utf-8') + b'\n.'
+        self.assertRaises(pickle.UnpicklingError, self.loads, pickled)
+
+
 class AbstractPicklerUnpicklerObjectTests(unittest.TestCase):
 
     pickler_class = None
diff --git a/Lib/test/test_pickle.py b/Lib/test/test_pickle.py
--- a/Lib/test/test_pickle.py
+++ b/Lib/test/test_pickle.py
@@ -7,6 +7,7 @@
 from test.pickletester import AbstractPickleTests
 from test.pickletester import AbstractPickleModuleTests
 from test.pickletester import AbstractPersistentPicklerTests
+from test.pickletester import AbstractIdentityPersistentPicklerTests
 from test.pickletester import AbstractPicklerUnpicklerObjectTests
 from test.pickletester import AbstractDispatchTableTests
 from test.pickletester import BigmemPickleTests
@@ -52,10 +53,7 @@
         return pickle.loads(buf, **kwds)
 
 
-class PyPersPicklerTests(AbstractPersistentPicklerTests):
-
-    pickler = pickle._Pickler
-    unpickler = pickle._Unpickler
+class PersistentPicklerUnpicklerMixin(object):
 
     def dumps(self, arg, proto=None):
         class PersPickler(self.pickler):
@@ -64,8 +62,7 @@
         f = io.BytesIO()
         p = PersPickler(f, proto)
         p.dump(arg)
-        f.seek(0)
-        return f.read()
+        return f.getvalue()
 
     def loads(self, buf, **kwds):
         class PersUnpickler(self.unpickler):
@@ -76,6 +73,20 @@
         return u.load()
 
 
+class PyPersPicklerTests(AbstractPersistentPicklerTests,
+                         PersistentPicklerUnpicklerMixin):
+
+    pickler = pickle._Pickler
+    unpickler = pickle._Unpickler
+
+
+class PyIdPersPicklerTests(AbstractIdentityPersistentPicklerTests,
+                           PersistentPicklerUnpicklerMixin):
+
+    pickler = pickle._Pickler
+    unpickler = pickle._Unpickler
+
+
 class PyPicklerUnpicklerObjectTests(AbstractPicklerUnpicklerObjectTests):
 
     pickler_class = pickle._Pickler
@@ -103,6 +114,10 @@
         pickler = _pickle.Pickler
         unpickler = _pickle.Unpickler
 
+    class CIdPersPicklerTests(PyIdPersPicklerTests):
+        pickler = _pickle.Pickler
+        unpickler = _pickle.Unpickler
+
     class CDumpPickle_LoadPickle(PyPicklerTests):
         pickler = _pickle.Pickler
         unpickler = pickle._Unpickler
@@ -128,7 +143,8 @@
 
 def test_main():
     tests = [PickleTests, PyPicklerTests, PyPersPicklerTests,
-             PyDispatchTableTests, PyChainDispatchTableTests]
+             PyIdPersPicklerTests, CIdPersPicklerTests, PyDispatchTableTests,
+             PyChainDispatchTableTests]
     if has_c_implementation:
         tests.extend([CPicklerTests, CPersPicklerTests,
                       CDumpPickle_LoadPickle, DumpPickle_CLoadPickle,
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -2856,26 +2856,33 @@
                 goto error;
         }
         else {
-            PyObject *pid_str = NULL;
-            char *pid_ascii_bytes;
-            Py_ssize_t size;
+            PyObject *pid_str;
+            PyObject *pid_ascii_bytes;
 
             pid_str = PyObject_Str(pid);
             if (pid_str == NULL)
                 goto error;
 
-            /* XXX: Should it check whether the persistent id only contains
-               ASCII characters? And what if the pid contains embedded
+            /* XXX: Should it check whether the pid contains embedded
                newlines? */
-            pid_ascii_bytes = _PyUnicode_AsStringAndSize(pid_str, &size);
+            pid_ascii_bytes = PyUnicode_AsASCIIString(pid_str);
             Py_DECREF(pid_str);
-            if (pid_ascii_bytes == NULL)
+            if (pid_ascii_bytes == NULL) {
+                if (PyErr_ExceptionMatches(PyExc_UnicodeEncodeError)) {
+                    PyErr_SetString(PicklingError, "persistent IDs in "
+                                    "protocol 0 must be ASCII strings");
+                }
                 goto error;
+            }
 
             if (_Pickler_Write(self, &persid_op, 1) < 0 ||
-                _Pickler_Write(self, pid_ascii_bytes, size) < 0 ||
-                _Pickler_Write(self, "\n", 1) < 0)
+                _Pickler_Write(self, PyBytes_AS_STRING(pid_ascii_bytes),
+                               PyBytes_GET_SIZE(pid_ascii_bytes)) < 0 ||
+                _Pickler_Write(self, "\n", 1) < 0) {
+                Py_DECREF(pid_ascii_bytes);
                 goto error;
+            }
+            Py_DECREF(pid_ascii_bytes);
         }
         status = 1;
     }
@@ -4668,9 +4675,14 @@
         if (len < 2)
             return bad_readline();
 
-        pid = PyBytes_FromStringAndSize(s, len - 1);
-        if (pid == NULL)
+        pid = PyUnicode_DecodeASCII(s, len - 1, "strict");
+        if (pid == NULL) {
+            if (PyErr_ExceptionMatches(PyExc_UnicodeDecodeError)) {
+                PyErr_SetString(UnpicklingError, "persistent IDs in "
+                                "protocol 0 must be ASCII strings");
+            }
             return -1;
+        }
 
         /* Ugh... this does not leak since _Unpickler_FastCall() steals the
            reference to pid first. */