diff --git a/rest_framework/compat.py b/rest_framework/compat.py
index 5fc6c6865..7664c4005 100644
--- a/rest_framework/compat.py
+++ b/rest_framework/compat.py
@@ -10,6 +10,14 @@ except ImportError:
     import StringIO
 
 
+def get_concrete_model(model_cls):
+    try:
+        return model_cls._meta.concrete_model
+    except AttributeError:
+        # 1.3 does not include concrete model
+        return model_cls
+
+
 # First implementation of Django class-based views did not include head method
 # in base View class - https://code.djangoproject.com/ticket/15668
 if django.VERSION >= (1, 4):
diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 986d42250..bdb0f10a4 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -4,6 +4,7 @@ import types
 from decimal import Decimal
 from django.core.serializers.base import DeserializedObject
 from django.utils.datastructures import SortedDict
+from rest_framework.compat import get_concrete_model
 from rest_framework.fields import *
 
 
@@ -299,7 +300,7 @@ class ModelSerializer(RelatedField, Serializer):
         else:
             cls = self.opts.model
 
-        opts = cls._meta.concrete_model._meta
+        opts = get_concrete_model(cls)._meta
         pk_field = opts.pk
         while pk_field.rel:
             pk_field = pk_field.rel.to._meta.pk
diff --git a/tox.ini b/tox.ini
index f9bd09909..bcfff6729 100644
--- a/tox.ini
+++ b/tox.ini
@@ -11,11 +11,11 @@ deps = https://github.com/django/django/zipball/master
 
 [testenv:py2.7-django1.4]
 basepython = python2.7
-deps = django>=1.4.1
+deps = django==1.4.1
 
 [testenv:py2.7-django1.3]
 basepython = python2.7
-deps = django>=1.3.2
+deps = django==1.3.3
 
 [testenv:py2.6-django1.5]
 basepython = python2.6
@@ -23,8 +23,8 @@ deps = https://github.com/django/django/zipball/master
 
 [testenv:py2.6-django1.4]
 basepython = python2.6
-deps = django>=1.4.1
+deps = django==1.4.1
 
 [testenv:py2.6-django1.3]
 basepython = python2.6
-deps = django>=1.3.2
+deps = django==1.3.3