AbstractThe relationship between two commonly co‐occurring filter‐feeders;Bosmina longirostrisandEudiaptomus gracilisat different nutrient levels, was investigated in bag experiments in Lake Gjersjøen. At the start of the experiment, the numerical relationship betweenBosminaandEudiaptomuswas less than 1 : 10, but during the experimental period, a strong increase inBosminaand a corresponding decrease inEudiaptomusand medium‐sized algae (5–25 μm) was observed. The shift fromEudiaptomustoBosminawas probably due to strong competition on mediumsized algae whileBosminain addition may utilize smaller algae and bacteria, not available toEudiaptomus. Their coexistence in the lake at much lower densities is probably due to a strong selective fish predation, suppressingBosmina.Daphnia longispina, not naturally occurring in the lake, showed strong competitive abilities in a fish