diff --git a/django/db/migrations/loader.py b/django/db/migrations/loader.py
index fac33c3ce7..b19c487e9f 100644
--- a/django/db/migrations/loader.py
+++ b/django/db/migrations/loader.py
@@ -124,6 +124,12 @@ class MigrationLoader(object):
                     app_config.label,
                 )
             if south_style_migrations:
+                if app_config.label in self.migrated_apps:
+                    raise BadMigrationError(
+                        "Migrated app %r contains South migrations. Make sure "
+                        "all numbered South migrations are deleted prior to "
+                        "creating Django migrations." % app_config.label
+                    )
                 self.unmigrated_apps.add(app_config.label)
 
     def get_migration(self, app_label, name_prefix):
diff --git a/docs/releases/1.8.6.txt b/docs/releases/1.8.6.txt
index 75bd79542b..76eb32902d 100644
--- a/docs/releases/1.8.6.txt
+++ b/docs/releases/1.8.6.txt
@@ -41,3 +41,6 @@ Bugfixes
 
 * Fixed crash with ``contrib.postgres.forms.SplitArrayField`` and
   ``IntegerField`` on invalid value (:ticket:`25597`).
+
+* Added a helpful error message when Django and South migrations exist in the
+  same directory (:ticket:`25618`).