Using return sludge in an oxidation ditch, the effect of oxygen concentration on the rates of denitratification and denitritification was investigated by the acetylene inhibition method during initial l-h incubation period after preculture under anaerobic conditions. The denitratification rate decreased as the oxygen concentration increased, and above 30 μM O 2, the reaction was suppressed completely. In contrast, denitritification occurred even under aerobic conditions. In the nitrite range of 70–7000 μM at temperature of 13°C, the denitritification rate under aerobic conditions was 5–20 μmol (g-VSS) −1h −1, which was 10–60% of that under anaerobic conditions. The inhibition of denitritification by oxygen seemed to conform to the simple competitive inhibition model.