To determine whether the quantity of food available to mosquitoes in their aquatic environment limits the effectiveness of microbial pathogens as biological control agents, experimentally well-nourished and malnourished larval Aedes aegypti (Linn.) were exposed to graded inocula of the entomopathogenic fungus, Metarhizium anisopliae. First instar larvae were provided access either to 3 or to 5 mg of food, and lots from each food regimen were inoculated with 20, 40, 60, or 80 µg of fungal spores/ml water. Application of the fungus to well-nourished larvae reduced the proportion developing to the adult stage, and increased the size of those adults that developed; their survival was not affected. In the case of malnourished larvae, such applications appeared not to effect the proportion of larvae that matured, and, paradoxically, increased the size and longevity of these resulting adults. By destroying a portion of the larvae, the pathogen apparently reduced competition among malnourished larvae, thereby enhancing their survival, as well as the size of the resulting adults. Thus, biocontrol agents may fail when used as larvicides against such nutrient-deprived mosquitoes as frequently occur in nature.