The oxidation of NH 4 + by Nitrosomonas europaea was insensitive to 10 mM NaClO 3 (sodium chlorate) but was strongly inhibited by NaClO 2 (sodium chlorite; K i , 2 μM). The oxidation of NO 2 − by Nitrobacter winogradskyi was inhibited by both ClO 3 − and ClO 2 − ( K i for ClO 2 − , 100 μM). N. winogradskyi reduced ClO 3 − to ClO 2 − under both aerobic and anaerobic conditions, and as much as 0.25 mM ClO 2 − was detected in the culture filtrate. In mixed N. europaea-N. winogradskyi cell suspensions, the oxidation of both NH 4 + and NO 2 − was inhibited in the presence of 10 mM ClO 3 − after a 2-h lag period, despite the fact that, under these conditions, ClO 2 − was not detected in the filtrate. The data are consistent with the hypothesis that, in mixed culture, NH 4 + oxidation is inhibited by ClO 2 − produced by reduction of ClO 3 − by the NO 2 − oxidizer. The use of ClO 3 − inhibition of NO 2 − oxidation in assays of nitrification by mixed populations necessitates cautious interpretation unless it can be shown that the oxidation of NH 4 + is not affected.