Our objective was to look at the effects of oxytetracycline (OTC), which is used in the fish farming industry to treat bacterial diseases, on the nitrification process in a simple aquatic system. The experiments were conducted in 9-1 aquaria containing sand substrate, synthetic freshwater, and active cultures of the nitrifying bacteria Nitrosomonas and Nitrobacter. The synthetic freshwater was amended with ammonia, at an initial concentration of approximately 5 mg/l NH sub(3)-N, to provide an energy source for the nitrifying bacteria. The effects of OTC were evaluated by measuring changes in the conversion of ammonia to nitrite and nitrate in reference to a negative control (0 mg/l OTC). We determined that nitrification was inhibited at all levels of OTC tested (12.5-75 mg/l). Concentrations of 50-75 mg/l OTC resulted in nearly complete inhibition of nitrification within 7 days. We estimated the 95% confidence interval for the 7-day EC50 to be 8.60-26.96 mg/l OTC. The potential for disrupting nitrification, which could lead to a build-up of toxic ammonia and nitrite, should be considered by aquaculturists treating diseased fishes.