The synthesis, activity and breakdown of the denitrifying enzymes of Rhizobium japonicum, R. lupini and R. meliloti were found to be regulated by O₂. Nitrogen oxide reductases were present in anaerobically grown and symbiotic R. japonicum, but in the case of organisms that had been grown aerobically the enzymes were induced only after a period of incubation under anaerobic conditions. Activity of the denitrification system that had been induced in aerobically grown cells was inhibited by O₂. Denitrification by anaerobically grown cells and bacteroids was stimulated by 5% O₂. Air inhibited denitrification completely. Little loss of denitrifying activity was shown by cells incubated in 5% O₂, but cells incubated at 10% O₂ showed a rapid loss of denitrification activity.