Experiments were conducted to determine the effects of various factors upon the rate of nitrate production by a stock culture of nitrifying bacteria. Actively metabolizing heterotrophic bacteria were found to have no effect upon the rate of nitrate production by the autotrophs. Under some circumstances, however, heterotrophs released materials into the medium which appeared to stimulate the autotrophs. Common domestic sewage was also found to have no effect upon the rate of nitrate production. When samples were removed from batch activated sludge reactors during the period of substrate removal and mixed with the autotrophs, no deleterious effects upon the rate of nitrate production were observed. The results suggest that unless specific inhibitory compounds are present in a wastewater, neither organic matter nor heterotrophic bacteria will have a deleterious effect upon the activity of autotrophic nitrifiers.