Description and identification is made of Gyrodactylus masu sp. n. collected from fins, body surfae, gilaments and gill arches of cultured salmonid flsh (Oncorhynchus masou, O. rhodurus and Salmo gairdneri) in Japan. The new species is most similar to G. derjavini Mikailav, 1975 in the shape of the hard parts, but different from it in the size of the hamuli and marginal hooks. Monthly monitoring of G. masu sp. n. on a single stock of Oncorhynchus masou was made during the period of May 1978 to Sep. 1979. There was a slight tendency that the hamuli and marginal hooks increased in length with decreasing water temperature and vice versa.