An adult male of Argulus coregoni Thorell, 1864 was collected from the skin of a gogi charr Salvelinus leucomaenis imbrius Jordan and McGregor (Salmoniformes: Salmonidae) in the uppermost reaches of the Takatsu River in Yoshiga, Shimane Prefecture, Japan. This finding represents the first record of A. coregoni from western Japan. Salvelinus leucomaenis imbrius is a new host for A. coregoni. This salmonid is endemic to rivers in part of the Chugoku Region, western Japan, and small isolated populations of the fish occurred at the sampling sites. No other salmonids had been stocked there. These facts indicate that A. coregoni is native to the river, which does not support the view put forward in the 1960’s that A. coregoni had been probably introduced from Europe into Japan.