To date, density-dependent effects on sexual selection have been studied only scantly. In this study we experimentally assessed the effect of breeding site density on the size distribution of mating convict cichlids Archocentrus nigrofasciatus in the field. We found that females were larger under low than high nest density. These results are better explained by density-dependent mate choice and mating competition than competing hypotheses of resource competition and predation pressure. We did not find differences in brood survival between the two nest density regimes. Nevertheless, convict cichlids avoided breeding in each other's close proximity, indicating that a high density of breeding pairs entails some other costs, such as energy loss through increased territorial aggression. Our results stress the importance of considering evolutionary effects of mate choice and mate competition in the context of availability of resources that determine the density of individuals that succeed to mate.