Ammonium-oxidizing bacteria were examined in two wetland environments, a freshwater marsh and an estuarine bay, during a 2-year period. Two predominant types were consistently isolated, one from each environment. Both isolates were identified as species of Nitrosomonas. Using a closed culture, high cell density assay, the effects of temperature, pH, salinity, Na+, K+, nitrite, nitrate, and ammonium concentrations on ammonium oxidation were determined. Maximum activity was observed for the freshwater isolate at 35C, pH 8.5, salinities of 0.3 to 0.5% Na+ and K+, and ammonium concentrations greater than 0.5 g/l. For the estuarine isolate, maximum activity was observed at 40C, pH 8.0, salinities of 0.5 to 1.0%, 1.0% Na+ and K+, and 0.2 g/l ammonium. The estuarine isolate had a Na+ requirement which could be partially substituted by the K+, suggesting that the organism is a true estuarine bacterium. Nitrite inhibited both isolates at concentrations greater than 5 mg/l, whereas nitrate had no significant effect on either isolate.