diff -r b09d07d1f696 Lib/gzip.py
--- a/Lib/gzip.py	Sat Aug 20 03:19:34 2011 +0200
+++ b/Lib/gzip.py	Sat Aug 20 10:09:25 2011 +0200
@@ -89,6 +89,12 @@
         return getattr(self.file, name)
 
 
+class BadGzipFile(IOError):
+    """
+    Exception raised if the processed gzip file is not valid.
+    """
+
+
 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.
@@ -247,10 +253,10 @@
             raise EOFError("Reached EOF")
 
         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)
@@ -442,10 +448,10 @@
         crc32 = read32(self.fileobj)
         isize = read32(self.fileobj)  # may exceed 2GB
         if crc32 != self.crc:
-            raise IOError("CRC check failed %s != %s" % (hex(crc32),
-                                                         hex(self.crc)))
+            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
diff -r b09d07d1f696 Lib/test/test_gzip.py
--- a/Lib/test/test_gzip.py	Sat Aug 20 03:19:34 2011 +0200
+++ b/Lib/test/test_gzip.py	Sat Aug 20 10:09:25 2011 +0200
@@ -306,6 +306,17 @@
             d = f.read()
             self.assertEqual(d, data1 * 50, "Incorrect data in file")
 
+    def test_gzip_exception(self):
+        self.assert_(issubclass(gzip.BadGzipFile, IOError))
+            
+    def test_bad_gzip_file(self):
+        with open(self.filename, 'wb') as file:
+            file.write(data1 * 50)
+        
+        with gzip.GzipFile(self.filename, 'r') as file:
+            self.assertRaises(IOError, file.readlines)
+            self.assertRaises(gzip.BadGzipFile, file.readlines)
+
     def test_non_seekable_file(self):
         uncompressed = data1 * 50
         buf = UnseekableIO()