Index: Lib/gzip.py
===================================================================
--- Lib/gzip.py	(revision 78997)
+++ Lib/gzip.py	(working copy)
@@ -45,6 +45,14 @@
     """
     return GzipFile(filename, mode, compresslevel)
 
+class BadGzipFile(IOError):
+    """
+    Exception raised if processed file seems not to be a valid gzip
+    archive.
+    """
+    pass
+
+
 class GzipFile(io.BufferedIOBase):
     """The GzipFile class simulates most of the methods of a file object with
     the exception of the readinto() and truncate() methods.
@@ -189,10 +197,10 @@
     def _read_gzip_header(self):
         magic = self.fileobj.read(2)
         if magic != b'\037\213':
-            raise IOError('Not a gzipped file')
+            raise BadGzipFile('Not a gzipped file')
         method = ord( self.fileobj.read(1) )
         if method != 8:
-            raise IOError('Unknown compression method')
+            raise BadGzipFile('Unknown compression method')
         flag = ord( self.fileobj.read(1) )
         self.mtime = read32(self.fileobj)
         # extraflag = self.fileobj.read(1)
@@ -343,10 +351,10 @@
         crc32 = read32(self.fileobj)
         isize = read32(self.fileobj)  # may exceed 2GB
         if crc32 != self.crc:
-            raise IOError("CRC check failed %s != %s" % (hex(crc32),
+            raise BadGzipFile("CRC check failed %s != %s" % (hex(crc32),
                                                          hex(self.crc)))
         elif isize != (self.size & 0xffffffff):
-            raise IOError("Incorrect length of data produced")
+            raise BadGzipFile("Incorrect length of data produced")
 
         # Gzip files can be padded with zeroes and still have archives.
         # Consume all zero bytes and set the file position to the first
Index: Lib/test/test_gzip.py
===================================================================
--- Lib/test/test_gzip.py	(revision 78997)
+++ Lib/test/test_gzip.py	(working copy)
@@ -264,6 +264,16 @@
         with gzip.GzipFile(self.filename, "rb") as f:
             d = f.read()
             self.assertEqual(d, data1 * 50, "Incorrect data in file")
+            
+    def test_bad_gzip_file(self):
+        file = open(self.filename, 'wb')
+        file.write(data1 * 50)
+        file.close()
+        
+        file = gzip.GzipFile(self.filename, 'r')
+        self.assertRaises(IOError, file.readlines)
+        self.assertRaises(gzip.BadGzipFile, file.readlines)
+        file.close()
 
 def test_main(verbose=None):
     support.run_unittest(TestGzip)