Species-specific differences in the responses of egg-laying mosquitoes (Diptera: Culicidae) to the presence of fish exudates in oviposition sites in laboratory bioassays have been related to the likelihood of encountering mosquito-eating fish in natural oviposition sites. We examined the responses of egg-laying mosquitoes to the presence of larvivorous fish in oviposition sites to test this hypothesis in the field. The number of Culex tarsalis Coquillett egg rafts laid on mesocosms (15.5 m2; 8 m3) containing caged mosquitofish (0.066 Gambusia affinis per liter) was reduced by 84% relative to mesocosms lacking fish. Egg-laying Culex quinquefasciatus Say did not differentiate significantly between comparatively small (0.11 m2) oviposition sites containing water conditioned with mosquitofish (0.3 G. affinis per liter) versus aged reservoir water. Cx. quinquefasciatus egg rafts were not collected from the larger mesocosms, and Cx. tarsalis egg rafts were rarely collected from the smaller oviposition sites. Oviposition preferences for characteristics of aquatic habitats that lack fish (e.g., small size, semipermanence, and high levels of organic enrichment in which hypoxic conditions are prevalent) may limit the coexistence of immature stages of some mosquito species such as Cx. quinquefasciatus and insectivorous fish. Mosquito species such as Cx. tarsalis that also use comparatively large water bodies as developmental sites may have evolved the ability to detect the presence of predatory fish.