Field collections of mosquitoes were made April-September 1977 and 1978 in the immediate vicinity of a hunting-dog kennel where the dogs had a chronic history of heartworm infection. Among 19 mosquito species taken in light-trap collections, only Aedes sticticus, Ae. vexans, and Anopheles punctipennis were found infected with what were presumed to be D. immitis larvae. Based on natural infection rates and the relative abundance of these three host species, An. punctipennis and Ae. sticticus appear to be among the most important vectors of dog heartworm at this site. Despite its greater abundance, Ae. vexans apparently plays a comparatively lesser role in D. immitis transmission owing to low natural infection rates and the development of only a few filariae per host. All Dirofilaria isolations were made in April, May, and June, suggesting that transmission may occur primarily during the spring and early summer in this area.