Mortality rates for test populations of Aedes aegypti were significantly increased with increases in concentrations of methoprene in the larval rearing media from 1.0 ppb (28% average mortality) to 10 ppb (84% average mortality). The mortality rates were not significantly changed when the protozoan parasite. Ascogregarina culicis, was used in combination with either concentration of methoprene against Ae. aegypti larvae. In contrast, mortality rates for Ae. epactius were not only significantly increased with increases in methoprene concentrations from 0.001 ppb (13% average mortality) to 0.01 ppb (53% average mortality), but also, the mortality rates at each IGR concentration were significantly higher when Ae. epactius larvae were first exposed to sporocysts of A. culicis and then to the IGR. Average mortality rates in this latter case ranged between 73 to 82%. The combined effects of A. culicis and methoprene on the mortality rates for Ae. epactius appear to be additive. Methoprene appears to have no significant effect either on the infectivity of the sporocyst stage of A. culicis or on the level of parasitism that can be established by this parasite in Ae. aegypti and Ae. epactius populations.