During the summers of 1976 and 1977. 3294 vertebrate host-seeking mosquitoes were captured by the human-bait method in areas where dog heartworm was known to occur. Following dissections of ovaries and other internal organs. specimens were age-graded by the Polovodova method and examined for filarial worms. “Sausage-stage,” late 2nd-stage, and (or) L3-stage filarial larvae were found in 46 (1.4%) of Aedes canadensis, Ae. excrueians, Ae. stimulans, Ae. sollieitans, Ae. vexans , and Psorophora ferox . Filarial worms occurred in nulliparous as well as parous mosquitoes. Although Ae. canadensis appears to be an important potential vector, the complete development of Dirofilaria immitis to the infectious stage in 3 other mosquito species suggests that mosquitoes of several species are probably involved in dog heartworm transmission in the northeastern United States.