Eight-hundred and forty eight dogs (480 females and 368 males) not on chemoprophylaxis were examined for Dirofilaria immitis infection from 2001 to 2002. Three-hundred and thirty nine (40.0%) of 848 samples tested with antigen detecting ELISA kits showed positive reaction for D. immitis antigen. More male (43.5%) than female (37.3%) dogs were affected in this study, although there was no significant difference between both groups. X 2 analysis showed that female and male dogs of shoreline areas had higher significant prevalence than other areas ( P<0.001), respectively. The mean overall positive rates of heartworm infection were 10.4% in dogs <2-year-old group, 46.5% in 2–4-year-old group, 48.4% in 4–6-year-old group and 50.3% in >6-year-old group. The older the age, the higher the prevalence of D. immitis infection in this study. X 2 analysis revealed a significant higher prevalence in 4–6-year-old group (100%) of shoreline areas and in 2–4-old-year group (51.2%) of urban areas (Chungnam province), respectively ( P<0.001), and also revealed that the over 6-year-old group (50.3%) in mean overall prevalence had the significant higher prevalence than other age group ( P<0.001). In climate comparison, our data showed that shoreline areas (69.5%) had the significant higher prevalence than urban areas and mountain areas ( P<0.001). This survey confirms that the prevalence of canine heartworm infection increases, and also that the prevalence in shoreline areas is higher than in other areas in South Korea.