Food overlap between roach R. rutilis and perch P. fluviatilis in a shallow eutrophic lake was low. Roach consumed largely detritus and algae not used by perch, but the competitive effect of the roach population on the perch population was substantial since the population size of roach was 10 times that of perch. Competitive interactions between the species is also supported by the fact that the roach population increased at the same time the perch population decreased. The size structure of the perch population changed slightly, but significantly. Increased interspecific competition probably results in a food niche contraction due to a decrease in individual growth rate and average size of the population. The same will be true when intraspecific competition increases in lakes with low habitat heterogeneity. Roach is more abundant than perch in eutrophic lakes; the opposite is true in oligotropic lakes. This is probably related to availability of detritus and algae, the size distribution of food resources and the distribution of food resources with respect to habitat.