diff --git a/mongoengine/django/auth.py b/mongoengine/django/auth.py
index 2711ee18..92424909 100644
--- a/mongoengine/django/auth.py
+++ b/mongoengine/django/auth.py
@@ -32,6 +32,12 @@ class User(Document):
     last_login = DateTimeField(default=datetime.datetime.now)
     date_joined = DateTimeField(default=datetime.datetime.now)
 
+    meta = {
+        'indexes': [
+            {'fields': ['username'], 'unique': True}
+        ]
+    }
+
     def __unicode__(self):
         return self.username