A male specimen of Argulus coregoni Thorell, 1864 was collected from the body surface of an ayu (Plecoglossus altivelis altivelis) (Salmoniformes: Plecoglossidae) collected in the Doi River, a tributary of the Niyodo River, Kochi Prefecture, Shikoku, Japan, on August 14, 2010. This collection represents the first locality record for A. coregoni in Shikoku, one of the four major islands of Japan.