Lake Biwa is the largest and ancient lake in Japan. The parasite fauna of aquatic animals of the lake has been extensively studied, but little information is available on the biology of fish-parasitic branchiurans. Two adult males of the argulid branchiuran Argulus ja- ponicus Thiele, 1900 were collected from the body surface of an individual of the three-lips, Opsariich- thys uncirostris uncirostris (Temminck and Schlegel, 1846), in Lake Biwa. This represents a new host record for A. japonicus and its first host record from the lake.

The three-lips, Opsariichthys uncirostris uncirostris (Cyprinidae), a new host of Argulus japonicus (Branchiura: Argulidae), with its first host record from Lake Biwa, Japan (Kazuya Nagasawa, Yuma Fujino and Hikaru Nakano)