Bacillus licheniformis has two pathways of arginine catabolism. In well-aerated cultures, the arginase route is present, and levels of catabolic ornithine carbamoyltransferase were low. An arginase pathway-deficient mutant, BL196, failed to grow on arginine as a nitrogen source under these conditions. In anaerobiosis, the wild type contained very low levels of arginase and ornithine transaminase. BL196 grew normally on glucose plus arginine in anaerobiosis and, like the wild type, had appreciable levels of catabolic transferase. Nitrate, like oxygen, repressed ornithine carbamoyltransferase and stimulated arginase synthesis. In aerobic cultures, arginase was repressed by glutamine in the presence of glucose, but not when the carbon-energy source was poor. In anaerobic cultures, ammonia repressed catabolic ornithine carbamoyltransferase, but glutamate and glutamine stimulated its synthesis. A second mutant, derived from BL196, retained the low arginase and ornithine transaminase levels of BL196 but produced high levels of deiminase pathway enzymes in the presence of oxygen.