Dosage-mortality tests conducted with preparasitic (infective) stage Romanomermis culicivorax Ross and Smith showed that insecticide concentrations of 0.0021 ppm chlorpyrifos, 0.018 ppm naled, 0.36 ppm propoxur, 1.06 temephos, 1.10 ppm fenthion, 2.95 ppm methoprene, 4.36 ppm diflubenzuron, and 6.78 ppm malathion caused a 50% loss of swimming activity and, consequently, impaired the ability of the nematode to locate and infect mosquito larvae. Chlorpyrifos was significantly (P<0.05) more toxic to the infective stage than the other insecticides tested. Malathion caused significantly (P<0.05) less mortality to the infective stage of the nematode than the other insecticides tested. The infective stage was significantly (P<0.05) less susceptible to all the insecticides tested, except naled, at the LC90 level than were 2nd instar Culex quinquefasciatus Say larvae. Naled and propoxur significantly reduced (P<0.05) the infectivity of the preparasitic stage at the LC50 response level established for 2nd instar Cx. quinquefasciatus larvae. Chlorpyrifos, malathion, fenthion, temephos, diflubenzuron and methoprene caused no significant reduction (P>0.05) of the infectivity of R. culicivorax at their respective LC50 response levels for 2nd instar Cs. quinquefasciatus larvae. Treatment of parasitized larvae, 24 hr after infection with insecticides, had no adverse effect on the viability of the postparasites that emerged. Juvenile nematodes, less than 7 days after emerging, molted to adults which mated and laid viable eggs after treatment with insecticides. The rate of eggs hatching after exposure to malathion, fenthion, and methoprene was significantly lower (P<0.05) than that of eggs exposed to propoxur and naled, however, no significant differences in percent egg hatch (P<0.05 existed between eggs treated with insecticides and the untreated eggs.