The significance of food concentration for selectivity was analyzed by video microscopy for 3 species of interception- feeding bacterivorous nanoflagellates of the genera Spumella, Ochromonas and Cafeteria. Inert beads and live bacteria were offered simultaneously at 5 different concentrations. The fate of individual prey particles was recorded during the stages of the particle-flagellate interaction capture, ingestion, digestion and egestion. The experiments revealed passive and active selection mechanisms that were regulated separately. Selective food uptake depended strongly on food concentration, whereas differential digestion was independent of the food concentration and independent of the number of previously ingested food particles. In addition to active selection of food items, passive selection occurred due to the different contact probabilities of prey. In contrast to the chrysomonads Spumella sp, and Ochromonas sp., the bicosoecid Cafeteria sp. showed no significant active selection, neither during food uptake nor during digestion. The results imply that it is more efficient for some interception-feeding flagellates to feed unselectively all particles that can be morphologically ingested and then to attempt to digest. these particles. Active selection in advance may only be efficient when the particle concentration is sufficiently high such that vacuole formation becomes time limiting.