Tadpoles and mosquito larvae often co-occur, and may compete for scarce resources. However, competition between such distantly related organisms has attracted less scientific attention than have interactions among closely related taxa. We examined ecological interactions in two tadpole-mosquito systems in southeastern Australia, one from freshwater ponds (Limnodynastes peronii and Culex quinquefasciatus) and one from brackish-water habitats (Crinia signifera and Ochlerotatus australis). Diets of these tadpoles and mosquito larvae overlap considerably, potentially leading to competition for food. Laboratory experiments show that, in both study systems, mosquitoes reduced the growth rates of tadpoles, and tadpoles reduced the growth rates and survival of mosquito larvae. These negative effects were seen even at high food levels. Thus, our study suggests that tadpoles and mosquito larvae affect each other strongly, and do so via pathways other than simple consumptive competition. Because mosquitoes are important vectors for human diseases, the global decline in amphibian populations may have more impact on human health than has generally been anticipated.