This study was carried out to outline the geographical distribution of the sweetfish which were infected by the metacercariae of Metagonimus yokogawai from August to October in 1979. The infection status was examined both by the muscle compression method and by digestion technique. The result was summarized as follows.

1. A total of 197 sweetfish, Plecoglossus altivelis was collected, 125 fish from 5 streams along the East Coast and 72 fish from 4 streams along the South Coast.

2. Total 125(63.5 percent) fish were infected by the metacercariae of M. yokogawai. The rate and intensity of infection in the south coastal areas were much higher than those in the east coastal areas; the rate was 42.4 percent in the East and 100 percent in the South, mean number of metacercariae per fish was 721 in the East and 14,308 in the South, and mean number of metacercariae per gram of flesh was 43 in the East compare with 904 in the South.

3. The sweetfish infected by the metacercaria of M. yokogawai were newly observed in 5 foci, i.e., Yeongogcheon in Myeongju Gun and Namdaecheon in Gangneung Si, Gangweon Do and Wangpicheon in Uljin Gun, Gyeongsangbug Do in the East, and a branch of Namgang in Euiryeong Gun and Sinhyeoncheon in Geoje Gun, Gyeongsangnam Do in the South. The rate of metacercarial infection was 4.1 percent in Yeongogcheon and 100 percent in others. The number of metacercariae in a fish ranged from 9 to 45,018 and the number of metacercariae per gram of flesh was the lowest in Gangneung Namdaecheon as 19 and the highest in Geoje Sinhyeoncheon as 1,237.

Summarizing these findings with previous records, M. yokogawai might be prevalent in the streams along the East Coast southern from Myeongju Gun, Gangweon Do and along the South Coast in Korea.