The present work which was investigated in July 1979, was to observe the wormicidal effects of the external mucous substances of 9 species of fresh water fishes (Cyprinus carpio, Parasilurus asotus, Anguilla japonica, Ophicephalus argus, Carassius carassius; golden crusian carp, Misgurnus anguillicaudatus, Zacco platypus, Pseudorasbora parva and Carassius carassius; crusian carp) on cercaria, liberated metacercaria and adult of Clonorchis sinensis. The mucous substances extracted by ether from the above 9 species of fishes were separated into many spots on the silica gel thin layer chromatography, and divided into many fractions in the silica gel column chromatography which used petroleum ether (30 percent) and chloroform (70 percent) as a solvent. On the silica gel thin layer chromatography of ether extracts from 9 species of fresh water fishes, each of the Rf. values which had wormicidal effect on the cercaria of C. sinensis were different from others; Rf. value of C. carassius (crusian carp) was 0.937, 0.709 in O. argus, 0.612 in A. japonica, 0.576 in P. asotus, 0.451 in C. carpio, 0.701 in Z. platypus, 0.385 in C. carassius(golden crusian carp) and 0.15 in P. parva. Time for wormicide was different from each other, too. It took 14 min. in a case of C. carassius(crusian carp), 25 min. in Z. platypus, 26 min. in C. carassius(golden crusian carp), 28 min. in C. carpio, 30 min. in P. asotus, 35 min. in O. argus, 40 min. P. parva and 180 min. in A. japonica. But any of the spots of M. anguillicaudatus did not show wormicidal effect on the cercaria within 10 hours. In the silica gel column chromatography of ether extracts from 9 species of fresh water fishes, the fractions which had wormicidal effect on the metacercaria of C. sienesis were different from each other; in a case of C. carassius(crusian carp), the first fraction only had wormicidal effect, the 2nd in P. asotus, the 3rd in O. argus, the 4th in C. carassius(golden crusian carp), the 2nd in C. carpio and the 4th in P. parva. Time for wormicide was 40 min. in C. carassius(crusian carp), 52 min. in P. asotus, 74 min. in C. carassius(golden crusian carp), 92 min. in O. argus, 95 min. in C. carpio and 140 min. in P. parva, but any of the fractions of A. japonica, M. anguillicaudatus and Z. platypus did not show wormicidal effect within 10 hours. The fractions which had wormicidal effect on the metacercaria of C. sinensis had also the same effect on the adult, but it took longer time to kill them. It took 3 hours in a case of C. carassius(crusian carp), 4 hrs. in C. carassius(golden crusian carp), 5 hrs. in P. asotus, 6 hrs. and 6 min. in O. argus, 6 hrs. and 10 min. in C. carpio and half and 6 hrs. in P. parva. But any of the fractions of A. japonica, M. anguillicaudatus and Z. platypus did not show wormicidal effect within 24 hrs.