The effect of protozoan grazing on nitrification rates under different conditions was examined. The spatial distribution of ammonia -and nitrite- oxidizing bacteria (AOB and NOB) in activated sludge was also examined using FISH/CSLM. Batch reactors were monitored for ammonia, nitrite, nitrate, and total nitrogen concentrations and bacterial numbers in the presence and absence of cycloheximide, a protozoan inhibitor. In the absence of protozoan grazing, rates of nitrification were lower than in batches with protozoa. Spatially, both AOB and NOB were found clustered within the floc and neither inhibiting the protozoa or inhibiting ammonia oxidation appeared to lower the amount of AOB and NOB present or their position. These results suggest that a reduction in protozoan grazing pressure allowed the heterotrophic bacteria to proliferate which caused a corresponding decrease in the rate of nitrification. These results suggest that AOB and NOB are less active in the absence of protozoa and indicates the role of protozoa in the cycling of nitrogen.