Argulus coregoni Thorell, 1864 was collected in August and September 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 collection represents the fourth and fifth records of A. coregoni from Ayu in the middle reaches of the river, where this fish species serves as an important host for A. corego- ni.