diff --git a/.gitignore b/.gitignore
index 69a9cd2a..65b4a5f5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -75,4 +75,4 @@ venv.bak/
 # local ignores
 .docker-tox/
 tests/sample-data/cds*.grib
-*.ix
\ No newline at end of file
+*.idx
\ No newline at end of file
diff --git a/Makefile b/Makefile
index b8f3559a..4b04735a 100644
--- a/Makefile
+++ b/Makefile
@@ -62,7 +62,7 @@ local-wheel:
 	$(PIP) wheel -e .
 
 testclean:
-	$(RM) -r */__pycache__ .coverage .cache tests/.ipynb_checkpoints *.ix tests/sample-data/*.ix
+	$(RM) -r */__pycache__ .coverage .cache tests/.ipynb_checkpoints *.idx tests/sample-data/*.idx
 
 clean: testclean
 	$(RM) -r */*.pyc htmlcov dist build .eggs
diff --git a/cfgrib/messages.py b/cfgrib/messages.py
index 57092159..ba91100a 100644
--- a/cfgrib/messages.py
+++ b/cfgrib/messages.py
@@ -209,7 +209,7 @@ def first(self):
         # type: () -> Message
         return next(iter(self))
 
-    def index(self, index_keys, indexpath='{path}.ix'):
+    def index(self, index_keys, indexpath='{path}.idx'):
         # (T.List[str], T.Optional[str]) -> FileIndex
         return FileIndex.from_indexpath_or_filestream(self, index_keys, indexpath)
 
@@ -251,7 +251,7 @@ def from_indexpath(cls, indexpath):
             return pickle.load(file)
 
     @classmethod
-    def from_indexpath_or_filestream(cls, filestream, index_keys, indexpath='{path}.ix'):
+    def from_indexpath_or_filestream(cls, filestream, index_keys, indexpath='{path}.idx'):
         # (FileStream, T.List[str], T.Optional[str]) -> FileIndex
         self = None
         if indexpath: