Planktonic bacteria may represent a substantial nutrient resource available to support the next higher trophic level in the microbial loop (heterotrophic flagellates). In this work we examined the utilization of different size classes of bacteria by flagellated protozoan predators of various sizes. The emphasis was to determine if prey‐size selection was a function of predator size. Pseudomonas sp. was grown in chemostats under conditions to yield “large” (mean size, 1.19 µm3) or “small” individuals (mean size, 0.36 µm3). Cells were fluorescently labeled (DTAF), mixed in various proportions, and fed to four protozoans ranging in size from 21 to 119 µm3. Bacteria in food vacuoles were enumerated and measured. Size distributions of both offered and taken prey were compared, and a selectivity index was calculated. Protozoans of all four sizes preferred large bacteria (between 0.8 and 1.2 µm3).