Gas–liquid chromatography (g.l.c.) was used for the analysis of certain metabolic end products of Actinomyces propionicus, as an aid in the separation of this organism from the morphologically similar Actinomyces species, A. israelii and A. naeslundii. Profiles of the chromatograms for the major volatile acids of five strains of A. propionicus studied were found to be distinct from those of four strains of A. israelii and four strains of A. naeslundii. The ratio of propionic acid to acetic acid was approximately 50 times as great for A. propionicus as for the other Actinomyces species. Formic acid was present in significant amounts in both A. israelii and A. naeslundii, but was present only in trace amounts in A. propionicus.Two major nonvolatile acids, lactic and succinic, were identified for the A. israelii and A. naeslundii strains. One of the A. propionicus strains also showed both acids in significant amounts; however, the other four strains of A. propionicus showed succinic acid in large amounts, but only trace amounts of lactic acid.