Nitrate and nitrite reduction under aerobic, microaerophillic, and anaerobic conditions was demonstrated in Spirillum lipoferum (ATCC 29145). Nitrite did not accumulated during assimilatory nitrate reduction in air. The nitrite produced during dissimilatory nitrate reduction accumulated in the medium but not in the cells. On exposure of the bacteria to nitrate and anaerobiosis, a low initial rate (lag) was followed by accelerated rates of nitrite accumulation. A 3-h anaerobic pretreatment, in the absence of nitrate, did not a void the lag phase. No nitrate reductase activity (NRA) developed in the presence of chloramphenicol. The data suggest that induction of anaerobic NRA in S. lipoferum required nitrate and protein synthesis. Anaerobic N2-ase by S. lipoferum was greatly stimulated in the presence of nitrate. The time course of nitrate reduction was coincidental with the pattern of nitrate-stimulated N2-ase activity inidcating that a relationship exists between these two processes.