Radiotracer studies confirm that the major products of glutamic acid metabolism byMicrococcus aerogenesare acetic acid, butyric acid, and carbon dioxide. When glutamic acid-1-C14was used as tracer the carboxyl carbons of both acetic and butyric acid were highly radioactive. Carbon 3 of butyric acid also contained some carbon-14. Glutamic acid-2-C14yielded acetic and butyric acid, both of which were highly labelled in carbon 2; carbon 4 of the butyric acid containing some carbon-14. All positions of acetic and butyric acid contained radioactivity when glutamic acid-3, 4-C14was used but carbons 3 and 4 of butyrate were much more highly labelled than carbons 1 and 2. Glutamic acid-5-C14was the only source of radioactive carbon dioxide.The data suggest that some of the butyric acid was derived from the intact four-carbon sequence which constitutes carbons 1, 2, 3, and 4 of glutamic acid and therefore, a pathway of glutamic acid degradation hitherto not recognized must occur inM.aerogenes.