In an attempt to clarify the epidemiological feature of C. sinensis in Mangyeong riverside area, the prevalence of clonorchiasis, infestation rate of the cercariae in Parafossarulus manchouricus, and detection rate of the metacercariae in fresh-water fishes were investigated from March 1 to September 30, 1983 at the upper, middle and lower reaches of the river. The results obtained were summarized as follows : 1. Detection rate of C. sinensis egg among inhabitants was 8.2% out of a total of 1,266 personsr, but the differences in detection rates were not statistically significant among upper, middle and lower reaches. According to sect, the detection rates were 10.3% in male and 6.1% in female (p<0.05), but by age groups, increases of the rates were observed as increase in age (p<0.05). 2. Out of a total of 380 fresh-water fishes of 32 different species, 93 fishes (25%) of 12 species were found positive with Clonorchis metacercariae, and there were differences in infection rates of the metacercariae among the fishes in 3 parts of the river; 11% in upper, 35% in middle, and 34% in lower reaches respectively. The metacercarial detection rates from various fishes were 97% in Pseudorasbora larva, Cultriculus eigenmanni (85%) , Gnathepogon strigatus (67%), Microphysogobio yaluensis (50%), Gnathopogon coreanus (47%), Pungtungia hergi (44%), Abbotting riwularis (40%), Moroco oxycephalus (33%), Coreoleuciscus splendidus (32%), Gnathepogon majimap (26%), Rhodeus ocellatus (7%), and Aphyocypris chinensis (3%) respectively. 3. Although very few p. manchouricus were collected at upper reach, 12 snails (0.7%) among a total of 1,713 were found infected with Clonorchis cercariae. Also the cercariae of Echinochasmup iapenicus (7.99%), Lexogenes liberum (0.99%), Cyathocotyle orientalis (0.75%), Esorchis oviformis (0.23%) and Asymphylodora japonica (0.05%) were detected from the snails.