Laboratory studies on the effect of Dirofilaria immitis on the egg production of the mosquito Aedes trivittatus revealed that D. immitis-infected mosquitoes produced significantly fewer eggs than uninfected mosquitoes. Egg production decreased as parasite burdens increased, but only mosquitoes harboring more than 15 D. immitis were severely affected. The ability of Ae. trivittatus to retain microfilariae within the blood clot seems beneficial to it by reducing parasite burdens, thereby increasing the mosquito's reproductive potential.