Conditions for the production of nitrite and nitrous oxide by an obligate methanotroph,Methylosinus trichosporium(OB 3b), were studied. The rate of nitrite productionwas correlated with the concentration of ammonia up to 20 mMin the presence of sufficient amounts of oxygen and inversely correlated with the amounts of methane in the system. The rate of nitrous oxide (N2O) productionwas correlated positively withand the amount of nitrite produced and inversely with the oxygen concentration in the system. Nitrite started to disappear when either oxygen or methane or both were depleted, but only a part of the loss could be accounted for by an increase in N2O. Maximum rates of nitrite and N2O production byMs.trichosporiumwere 6.9 ×5 1016and 2.2 × 10−17 mol∙cell−1∙day−1, respectively. These values are about 0.2 and 1.6% of the values forNitrosomonas europaea. Therefore, production of nitrite and N2O by methanotrophs in aquatic environments may not be as significant as that ofNitrosomo