We made an ecological and morphological study of a species belonging to Benedenia, and investigated a parasitic influences and preventive method on the fish. The results obtained are as follows: 1) The species of Benedenia were collected at Hibiki-nada of the coast of Yamaguti-ken, being parasitic on Fugu vermieularis, Fugu niphobles, Fugu pardalis, Fugu vermicularis radiatus, Epinephelus moara and Semicossyphus reticulatus. 2) As the Benedenia was experimentally parasitized on Echeneis naucrates, Ostracion tubereculatus and Coniistius zonatus, it was found that they were not parasitic on the specific fish.