diff --git a/xray/id9.py b/xray/id9.py
index a71886b..221b6ed 100644
--- a/xray/id9.py
+++ b/xray/id9.py
@@ -10,7 +10,7 @@ from . import utils
 from . import storage
 from . import filters
 
-default_extension = ".npz"
+default_extension = ".h5"
 
 def _conv(x):
     try: