The end products of the metabolism of phenylalanine, tyrosine and tryptophan by growing cultures of clostridia have been identified. The species used were Clostridium aminovalericum; C. bifermentans; C. botulinum proteolytic type A; C. botulinum proteolytic type B; C. cochlearium; C. difficile; C. ghoni; C. histolyticum; C. lentoputrescens; C. limosum; C. lituseburense; C. malenomenatum; C. mangenoti; C. propionicum; C. putrefaciens; C. sordellii; C. sporogenes; C. sporosphaeroides; C. sticklandii; C. subterminale; C. tetani; C. tetanomorphum. The mixture of aromatic compounds formed, which depended upon the species, included phenyl acetic acid, phenyl propionic acid, phenyl lactic acid, phenol, p-cresol, p-hydroxy phenyl acetic acid, p-hydroxy phenyl propionic acid, indole, indole acetic acid and indole propionic acid.