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1. We tested experimentally for interactions between mosquito larvae, Culiseta longiareolata, and green toad immatures, Bufo viridis, the two most abundant macrospecies of temporary pools in the Negev Desert, Israel. Since both appear to feed largely on periphyton, and since they co-occur in extremely high densities, competition for food may occur.

2. In artificial outdoor pools, we followed the fate of first-instar Culiseta larvae (50 per litre) and early-stage tadpoles (5 per litre) in the presence or absence of the other species over 1 month. These densities were well within the range of densities found in natural pools. We replicated each treatment five times.

3. Survival of neither species was affected by the other species. However, time to pupation in Culiseta was longer and pupal size was smaller in the presence of Bufo tadpoles. Similarly, tadpole weights were lower and the percentage of Bufo reaching metamorphosis by the end of the experiment was lower in the presence of Culiseta larvae. We attribute these mutual negative effects upon each other to interspecific competition.

4. Late-stage Culiseta larvae also preyed upon Bufo hatchlings in both field enclosure and laboratory experiments. This is the first demonstration of a mosquito larva as a natural predator of a vertebrate. The high predation rates indicate that Culiseta affects Bufo populations significantly via predation in nature.

5. These two surprising roles of a mosquito larva-a competitor and predator of a vertebrate-emphasize that how communities are organized may be misinterpreted by presuming particular roles for species based on taxonomic affiliation.