Around the time of his famous conversion in 386, Augustine planned to dedicate a treatise to each of the artes liberales. However, he finished only a work on grammar and the first part of the De musica (books I-VI), which deals with rhythmus\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. The introduction includes a full analysis of the manuscript tradition from the 8th to the 14th century and a selective analysis of the later manuscript tradition. Among the conclusions reached are that all extant manuscripts descend from a single archetype which is not identical with the original text, that most manuscripts can be divided into four families, and that the text can be established on the basis of six of the oldest manuscripts. The introduction also contains a discussion of the much-debated question concerning Augustine's own revision of the sixth book, a section where solutions are proposed to several textual problems that were confronted during the establishment of the text, and a selective commentary on the contents of the sixth book of the De musica.