Mosquitofish, Gambusia affinis, survived in an impounded water area within an underground storm drain system in Fresno, CA through a 14-wk period during the summer of 1982. The number and relative condition (K-factor) of the stocked fish slowly declined with the depletion of the food supply, as determined by fish gut content analysis, at the tenth week after introduction. However, the mosquitofish were effective in reducing the number of adult Culex quinquefasciatus produced in the drain system. Reductions of 75, 89 and 94% below those of an untreated control area were obtained after the first, second and third month, respectively. While female mosquitofish (gravid before introduction) produced offspring, no mating of fish within the drain was found.