The effects of seven thiophosphorus pesticides and their degradation products on estuarine ammonium oxidizers were examined. Using an axenic, closed-culture, high cell density assay, the pesticides caused little inhibition of ammonium oxidation. However, the degradation product of methyl parathion, p-aminophenol, significantly inhibited ammonium oxidation in levels as low as 0.01 mg/L. Estuarine sediments containing indigenous ammonium oxidizers were exposed to the pesticides over a 14-day period and activity of the ammonium oxidizers were inhibited after 14 days. Three pesticides were aged in estuarine sediments under aerobic, anaerobic, and microaerophilic conditions, and axenic cultures of ammonium oxidizers were exposed to the resulting metabolities.