Adult female mosquitoes were collected in the Lansing, Michigan area during the summers of 1974 and 1975 in dog-baited traps and CDC miniature light traps. Mosquitoes were identified and placed in groups of 25 or 30, crushed in 0.09% saline, and examined for the presence of Dirofilaria immitis (Leidy) larvae. Presumed D. immitis infective larvae were isolated from field-captured Aedes vexans (Meigen) on 3 occasions and Anopheles quadrimaculatus Say on 2 occasions D. immitis developmental trails were conducted in the laboratory using Michigan strains of Ae. stimulans (Walker, Ae. vexans, An. quadrimaculatus, Coquillettidia perturbans (Walker) and Culex pipiens Linnaeus. Complete development of D. immitis to the infective stage was noted in Ae. vexans, An. quadrimaculatus, and Cx. pipiens. Ae. vexans and An. quadrimaculatus are considered to be the primary vectors of D. immitis in the study area. Cx. pipiens is a possible natural vector.