A wind tunnel device was utilized in laboratory tests to determine the susceptibility to 6 selected insecticides of natural populations of adult females. Culex pipiens quinquefasciatus Say collected from 6 Southern Louisiana parishes. Mosquitoes collected from locations having mosquito control programs were significantly less (P<0.05) susceptible to the organophosphate insecticides temephos, chlorpyrifos, fenthion, malathion and naled than those populations from areas having no mosquito control programs. The C. p quinquefasciatus adults exposed to propoxur showed little variation in susceptibility between the populations collected from the 6 locations. Propoxur caused excellent knockdown and high mortality in 1-hour knockdown test on mosquitoes collected from all areas sampled in this study. Naled caused excellent knockdown and high mortality to C. p. quinquefasciatus populations collected from areas where no organophosphate tolerance existed.