Index: django/db/models/fields/__init__.py
===================================================================
--- django/db/models/fields/__init__.py (revision 5377)
+++ django/db/models/fields/__init__.py (working copy)
@@ -838,6 +838,11 @@
     def get_manipulator_field_objs(self):
         return [oldforms.PositiveIntegerField]
 
+    def formfield(self, **kwargs):
+        defaults = {'min_value': 0}
+        defaults.update(kwargs)
+        return super(PositiveIntegerField, self).formfield(**defaults)
+
 class PositiveSmallIntegerField(IntegerField):
     def get_manipulator_field_objs(self):
         return [oldforms.PositiveSmallIntegerField]