Anopheles dirus species A and B individuals were exposed as 4th-stage larvae to sublethal concentrations of methoprene. Median lethal concentrations were 0.21 ppb for species A and 0.17 ppb for species B. When exposed to 0.10 ppb methoprene the sex ratio of species. A changed from fewer males to more males; no effect was observed in the sex ratio of species B. Exposure to methoprene had no effect on wing length and survival of either species. Results indicate that exposure to methoprene significantly affected fecundity of both species of An. dirus.