The status of Dirofilaria immitis infection was assessed in pet dogs of Busan, Korea, and chemoprophylactic effects of microfilaricidal medication were evaluated. A total of 294 pet dogs older than 6 mo were examined, 217 of which had been maintained indoors, and 77 had been kept outdoors. The $Snap^R$ kit and direct microscopic examinations of the peripheral blood were used. The mean overall parasite positive rates were 10.2% and 6.5%, respectively. Outdoor dogs evidenced adult worm infection rate of 31.2% and microfilaria infection rate of 18.2%. The indoor dogs, however, evidenced adult worm infection rate of 2.8% and microfilaria infection rate of 2.3%. The prevalence in males was more than 2 times that of females. The changing pattern of infection rates by age evidenced a gradual increase, from 2- to 6-year-old dogs, after which, a decrease in infection rates was noted. With regard to chemoprophylaxis, the infection rates of complete and incomplete chemoprophylaxis groups were found to be 2-3 times lower than that of the non-chemoprophylaxis group. The results of the present study indicate that the risk of exposure to D. immitis in pet dogs is quite high, particularly in male outdoor dogs, and chemoprophylactic measures were quite effective.