In July, 1973, it was observed that many young carps in many farm ponds in Nagano, Yamagata and Akita prefectures were infected with a kind of cestode belonging to the genus Bothriocephalus. About 100 cestode specimens of different stages were collected from the intestines of 4 fishes (body weight : 20.7-31.6g). Ten of the largest size group among the mature worms obtained were examined morphologically and identified with B. opsariichthydis YAMAGUTI, 1934.
The criteria for identification were as follows.: 1) As regards the shape and the structure of the body, the present species resembled to B. acheilognathi YAMAGUTI, 1934. B. opsariichthydis YAMAGUTI, 1934, B. fluviatilis YAMAGUTI, 1952 and B. gowkongensis YEH, 1955 which had all been found in the cyprinoid fish in Japan and China. 2) The present species differed from B. gowkongensis in the point that the egg was never embryonated when laid. 3) As regards the body length the present species (17.3 cm in average) was rather close to B. opsariichthydis (more than 10 cm long according to YAMAGUTI), but differed from B. acheilognathi (about 8 cm) and B. fluviatilis (2.2 cm). 4) It was thought that B. acheilognati and B. fluviatilis were young stages of B. opsariichthydis.