Dirofilaria immitis, the canine heartworm, is one of the most important filaroids parasite in carnivores. The dog and its close relative are natural hosts, but infection also occurs in cat, wild carnivores and man. Adult heartworms normally are found in the pulmonary arteries. Female kind of Dirofilaria immitis frees its microfilaria in blood stream and microfilaria is distributed by circulatory system throughout the body. Several species of mosquitoes are as intermediated hosts. This study was done within 8 months in six areas susceptible to infection in the vicinity of Tabriz. 60 dogs (between 1-4 years old) were caught by strychnine poisoned meat and were necropsied. In examination of hearts of dogs, Dirofilaria immitis is found in 19 cases (31.6%) and all worms were located in right ventricle and in right arteries. There were no worms in pulmonary arteries. In positive cases at least 1 worm and at most 18 worms were observed. The intensity of infection in old dogs was much greater than those of young dogs. Because of high prevalence of Dirofilaria immitis in dogs and the risk of human infection, a control program must be carried out for controlling of this worm and its intermediate host.