Grazing rates of mixed cultures of freshwater, heterotrophic nanoflagellates on two populations of bacterial prey present together at varying concentrations were measured by using fluorescently labeled bacteria. The effect of one population on the ingestion kinetics of the other was consistent with a theory based on competitive inhibition of enzymatic reactions. However, allochthonous bacteria, when present in low concentrations within a much larger population of small autochthonous bacteria, may be preferentially grazed, which is due to their large size.