This is a PhD dissertation. Around the time of his famous conversion in 386, Augustine planned to dedicate a treatise to each of the artes liberals. However, he finished only a work on grammar and the first part of the De Musica, which deals with rythmus; the second part, which was to treat melos, was never written, since Augustine became occupied with his ecclesiastical career. The present work is the first critical edition of the sixth book of the De musica; the latin text is accompanied by an English translation