""" The compression level must be Z_DEFAULT_COMPRESSION, or between 0 and 9: 1 gives best speed, 9 gives best compression, 0 gives no compression at all (the input data is simply copied a block at a time). Z_DEFAULT_COMPRESSION requests a default compromise between speed and compression (currently equivalent to level 6). """ Z_DEFAULT_COMPRESSION is defined as (-1), and appears to be the value used if --compress is specified without --compress-level, making the default functionally equivalent to 6. -- Best, Ben