Tadpoles and mosquito larvae compete for scarce resources in natural freshwater ecosystems, but factors determining the extent of competition between these two groups of organisms remain largely unstudied. Natural ponds display great variation both in the densities of larvae, and in the body sizes of tadpoles. We set up replicated artificial pond experiments to examine the effects of population density and tadpole size on interactions between tadpoles and mosquito larvae. We examined the effects of larval density in two systems of co-occurring tadpoles and mosquito larvae, one from brackish-water ephemeral ponds (Crinia signifera with Ochlerotatus australis) and one from permanent freshwater ponds (Limnodynastes peronii with Culex quinquefasciatus). In both systems, increasing densities of larvae suppressed growth and development both of conspecifics and of the competing taxon. In the C. quinquefasciatus-L. peronii system, larger tadpoles exerted more powerful suppression. Our results suggest that mosquito developmental rates and adult body sizes (and thus, the danger which mosquitoes pose to public health) may be reduced if natural water-bodies contain dense populations of large tadpoles.