diff --git a/warc/arc.py b/warc/arc.py
index 5889587..43a8f70 100644
--- a/warc/arc.py
+++ b/warc/arc.py
@@ -251,6 +251,14 @@ def __init__(self, filename=None, mode=None, fileobj=None, version = None, file_
         """
         if fileobj is None:
             fileobj = __builtin__.open(filename, mode or "rb")
+            mode = fileobj.mode
+        # initialize compress based on filename, if not already specified
+        if compress is None and filename and filename.endswith(".gz"):
+            compress = True
+
+        if compress:
+            fileobj = gzip2.GzipFile(fileobj=fileobj, mode=mode)
+
         self.fileobj = fileobj
 
         if version != None and int(version) not in (1, 2):