An epidemiological investigation was conducted on Metagonimus yokogawai infection in Hadong Gun (County), Gyeongsang Nam Do, Korea, and following findings were obtained.

1. Overall prevalence rate of Metagonimus yokogawai infection was 29.1 percent from a total of 1,163 inhabitants examined. By area of river and stream, Seomjin-gang R.: 9.5 percent (out of 95 inhabitants examined) in Jeondo Ri in the lower river, 19.9 percent (156) at Dugog Ri near Hadong town, and 90.8 percent (65) in Tab Ri of Hwagae Myeon where Hwagae-cheon Stream joins; Hwagae-Cheon Stream: 42.2 percent (45) in Yonggang Ri of the middle stream and 27.5 percent (72) in Daeseong Ri of the upper stream; Hoengcheon-gang R.: 54.7 percent (161) in Gojeol Ri of the lower river, 40.6 percent (128) in Hoengcheon Ri of the middle stream, and 5.4 percent (93) in Jungi Ri of the upper stream, and 43.8 percent (89) in Uge Ri located along a tributary of the Hoengcheon-gang R. and 4.0 percent (101) in Jangam Ri which is located about 4 km away from the river; Deogcheon-gang R.: 18.5 percent (54) in Munam Ri; Gwangog-cheon Stream: 2.8 percent (107) in Gwangog Ri.

2. The difference in prevalence rate of the infection by area was correlated with the opportunity and amount of raw eating of sweetfish by the inhabitants in the respective areas.

3. By age and sex, the overall prevalence rate increased from young age group and reached a peak at 20-29 age group and then the rate decreased gradually. In high endemic areas, however, a peak level of the rate lasted up to 40-49 age group in male. On the other hand, in low endemic areas, the rate was significantly lower in female, though moderate level of the infection was seen in male. The overall prevalence rate was higher in male than in female and the difference was greater in middle age groups.

4. The most significant mode of infection by the inhabitants was the raw eating of sweetfish which commonly harbour metacercariae of M. yokogawai during summer through autumn. Besides, it was noted that the infection can be acquired through contamination with the metacercariae on food of various side-dish and kitchen utensils as well as on hands of a cook when such fish are prepared or handled.

5. The inhabitants usually begin to acquire infection from early June of a year when fishing of sweetfish opens and acquire infection more frequently in July and August when fish are caught more productively and favored for raw consumption by inhabitants. From September, sweetfish are no more favored for raw consumption and accordingly are dried and cooked in this season.

6. The infection in the sweetfish Plecoglossus altivelis as the second intermediate host was first observed from May with low level of infection and the intensity of infection in a fish rapidly increased until reaching a peak in October when sweetfish season normally ends.

7. The first intermediate host snail of M. yokogawai in this area was Semisulcospira coreana (Martens) which is widely distributed in the streams and rivers in this locality.