An adult female of Argulus coregoni Thorell, 1864 was collected in mid-July 2019 from the body surface of Ayu, Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846), in the middle reaches of the Nagara River, Gifu Prefecture, central Japan. This represents the second record of A. coregoni from the same collection site, from which the species was collected in late June 2013 from the same host species. Argulus coregoni is considered to parasitize both native and artificially released individuals of Ayu from spring to late autumn in the middle reaches of the river.