Glutamine synthetase, glutamate synthase and alanine dehydrogenase activities were detected in crude extracts of . Glutamate dehydrogenase and alanine: 2-oxoglutarate aminotransferase could not be found. Glutamate synthase levels were independent of the nitrogen source in the culture medium. Glutamine synthetase activity was repressed in the presence of ammonium, and a rapid inactivation of the enzyme was seen after ammonium shock. Alanine dehydrogenase was induced by alanine or ammonium concentrations higher than 20 m. Glutamate and, in lower amounts, glutamine, alanine, ?gM-aminobutyrate and aspartate were the main constituents of the intracellular pool of free amino acids. Only the pool concentrations of glutamine and alanine were markedly influenced by the nitrogen source used for growth. Mutants devoid of glutamine synthetase, glutamate synthase, or alanine dehydrogenase were isolated. The pattern of utilization of nitrogen sources by the three classes of mutants indicated that the glutamine synthetase–glutamate synthase pathway is the only means of assimilation of ammonium in . Alanine dehydrogenase is not directly involved in that process, but a possible related function is discussed.