diff --git a/download.py b/download.py
index 976e68d..814a00e 100644
--- a/download.py
+++ b/download.py
@@ -2,6 +2,7 @@
 import sys
 import cgi
 import os
+import os.path
 import datetime
 
 from do_authentication import authenticate
@@ -118,9 +119,10 @@ def download_one_zone(url, output_directory):
         # This is where the zone file will be saved
         path = '{0}/{1}'.format(output_directory, filename)
 
-        with open(path, 'wb') as f:
-            for chunk in download_zone_response.iter_content(1024):
-                f.write(chunk)
+        if not os.path.isfile(path):
+            with open(path, 'wb') as f:
+                for chunk in download_zone_response.iter_content(1024):
+                    f.write(chunk)
 
         print("{0}: Completed downloading zone to file {1}".format(str(datetime.datetime.now()), path))