Growth and reproduction of three different-sizedDahniaspecies were measured under increasing competitive conditions. Only when food was not limiting, the largest speciesD. magnawas superior to both smaller species with respect to total production and production of offspring. This advantage disappeared when the availability of food decreased. Under limiting conditions all three species had similar rates of production; thus, they utilized the available food resources with the same efficiency. Finally, at the lowest food concentrationD. magnawas no longer able to reproduce, but the smallest species,D. longispina, still produced eggs. Therefore, contrary to the “Size Efficiency Hypothesis.” the smallest species was the best competitor.