diff --git a/sphfile/sphfile.py b/sphfile/sphfile.py
index 6ab55dd..1075526 100644
--- a/sphfile/sphfile.py
+++ b/sphfile/sphfile.py
@@ -128,7 +128,7 @@ def write_wav(self, filename, start=None, stop=None):
                 data = self.time_range(start, stop)
             else:
                 data = self.content
-            fh.writeframes(data.tostring())
+            fh.writeframes(data.tobytes())
         return filename
 
     def write_sph(self, filename, start=None, stop=None,extra_headers=None):