Development of Dirofilaria immitis to the 3rd infective state in the mouth-parts of Aedes triseriatus was first noted at constant temperatures of 30, 26, 22 and 18 C after 8, 11, 17 and 29 days post-infection respectively, and in Ae. vexans after 12 days at 26 C. In infected Ae. triseriatus held at 16, 14 and 12 C only partial or no development occurred. In infected Ae. triseriatus held at 16 C for 12 or 22 days development of D. immitis to the infective stage occurred when mosquitoes were transferred to 26 C. Pigmental encapsulation of D. immitis was observed more frequently in Ae. vexans than in Ae. triseriatus. Mortality in infected mosquitoes was apparently higher than in unaffected mosquitoes at all temperatures investigated.