Groups of Culex quinquefasciatus were exposed as fourth instar larvae to sublethal concentrations (0.1 LC50 and LC50) of malathion, methoprene, propoxur or resmethrin. Females exposed as larvae to an LC50 level of methoprene had reduced wing length and longevity. Egg production was reduced by 50% and 39% in those mosquitoes exposed to LC50 levels of malathion or methoprene, respectively. In contrast, egg production and egg raft size increased following treatment with 0.1 LC50 levels of malathion or methoprene. Females exposed as larvae to methoprene laid 30% fewer eggs per raft, and egg hatching decreased 36% compared with controls. Females exposed as larvae to LC50 levels of malathion or methoprene laid shorter eggs than controls. The proportion of females in the adult population was reduced following exposure to either propoxur or resmethrin, and increased following exposure to malathion. Time to pupation and time of emergence of the adult populations were increased following exposure to most of the insecticidal treatments. These results indicated that a single, sublethal exposure to certain insecticides had a significant effect on mosquito reproduction.