Despite its ecological importance, essential aspects of microbial N2O reductionsuch as the effect of O-2 availability on the N2O sink capacity of a communityremain unclear. We studied N2O vs. aerobic respiration in a chemostat culture to explore (i) the extent to which simultaneous respiration of N2O and O-2 can occur, (ii) the mechanism governing the competition for N2O and O-2, and (iii) how the N2O-reducing capacity of a community is affected by dynamic oxic/anoxic shifts such as those that may occur during nitrogen removal in wastewater treatment systems. Despite its prolonged growth and enrichment with N2O as the sole electron acceptor, the culture readily switched to aerobic respiration upon exposure to O-2. When supplied simultaneously, N2O reduction to N-2 was only detected when the O-2 concentration was limiting the respiration rate. The biomass yields per electron accepted during growth on N2O are in agreement with our current knowledge of electron transport chain biochemistry in model denitrifiers like Paracoccus denitrificans. The culture's affinity constant (K-S) for O-2 was found to be two orders of magnitude lower than the value for N2O, explaining the preferential use of O-2 over N2O under most environmentally relevant conditions.