diff --git a/hooks/percona_hooks.py b/hooks/percona_hooks.py
index 4b4fdec..22331da 100755
--- a/hooks/percona_hooks.py
+++ b/hooks/percona_hooks.py
@@ -520,7 +520,8 @@ def db_changed(relation_id=None, unit=None, admin=None):
         return
 
     db_name, _ = (unit or remote_unit()).split("/")
-    username = db_name
+    # mysql has a username length limit.
+    username = db_name[:16]
     db_helper = get_db_helper()
     addr = relation_get('private-address', unit=unit, rid=relation_id)
     password = db_helper.configure_db(addr, db_name, username, admin=admin)