diff --git a/boututils/datafile.py b/boututils/datafile.py
index ac23bf2..76dd318 100644
--- a/boututils/datafile.py
+++ b/boututils/datafile.py
@@ -82,7 +82,7 @@ class DataFile(object):
     impl = None
 
     def __init__(
-        self, filename=None, write=False, create=False, format="NETCDF3_64BIT", **kwargs
+        self, filename=None, write=False, create=False, format="NETCDF4", **kwargs
     ):
         """