Incubation of whole cells of the nitrifying bacterium Nitrosomonas europaea with ethylene led to the formation of ethylene oxide. Ethylene oxide production was prevented by inhibitors of ammonium ion oxidation, and showed properties implying that ethylene is a substrate for the ammonia oxidising enzyme, ammonia monooxygenase. Endogenous substrates, hydroxylamine, hydrazine and ammonium ions were compared as sources of reducing power in terms of rates and stoichiometries of ethylene oxidation. The highest rates of ethylene oxide formation (15 μmol h-1 mg protein-1) were obtained with hydrazine as donor. The data suggest that at high concentrations of ethylene the rate of oxidation is limited by the rate at which reducing power can be supplied to the monooxygenase, not by an intrinsic V max. Ethylene had an inhibitory effect on the rate of ammonium ion utilisation; an approximate K i of 80 μM was derived, but the results deviated from simple competitive behaviour. Measurement of relative rates of ethylene oxide formation and ammonium ion utilization led to a k cat/K m value for ethylene of 1.1 relative to NH 4 + , or 0.04 relative to the true natural substrate, NH3. The effects of higher concentrations of ethylene oxide on oxygen uptake rates were also investigated. The results imply that ethylene oxide is also a substrate for the monooxygenase, but with a much lower affinity than ethylene.