In this study, various amounts of oxygen were added to denitrifying chemostats receiving benzoate to mimic the input of oxygen to anoxic zones of biological nutrient removal systems. The effect of oxygen on the biodegradative capability of the mixed-microbial culture for benzoate was investigated. The anoxic benzoate biodegradative capability of the culture was not significantly changed as the mass flowrate of oxygen was increased to 40% of the input benzoate chemical oxygen demand (COD) mass flowrate, but was decreased approximately 70% when the mass flowrate of oxygen was increased to 70% of the input benzoate COD mass flowrate. The decrease in the anoxic benzoate biodegradative capability was due primarily to the loss of the denitrifying enzymes (measured by the anoxic pyruvate-degrading ability) and not to the loss of the key anoxic catabolic enzyme (benzoyl-coenzyme A reductase). The proportional increase in the concentration of nitrate as the residual terminal electron acceptor and the lack of synthesis of aerobic ring-cleavage enzymes as the oxygen input to the chemostat was increased suggest that the mixed microbial culture preferred oxygen to nitrate as the terminal electron acceptor, but degraded benzoate using the anoxic. metabolic pathway. The concentration of the mixed microbial culture increased as the oxygen input to the chemostat was increased, suggesting that the oxygen was used by cytochrome cbb(3) rather than quinol oxidase because the energetic yield of cytochrome cbb3 is higher than that of quinol oxidase or the nitrogen oxide reductases.