Mpg321 is a clone of the popular mpg123 command-line mp3 player. It should function as a drop-in replacement for mpg123 in many cases. While some of the functionality of mpg123 is not yet implemented, mpg321 should function properly in most cases for most people, such as for frontends such as gqmpeg.

mpg321 is based on the mad MPEG audio decoding library. It therefore is highly accurate, and also uses only fixed-point calculation, making it more efficient on machines without a floating-point unit. It is not as fast as mpg123 on systems which have a floating point unit.