An adult female of the argulid branchiuran Argulus coregoni Thorell, 1864 was collected from the dorsal body surface of an oily bitterling, Tanakia limbata (Temminck and Schlegel, 1846) (Cyprinidae: Acheilognathinae), in the lower reaches of the Asahi River, Okayama Prefecture, western Japan. This represents a new host record for A. coregoni and its new prefectural record in Japan. The external morphology of the female collected is reported in detail. Tanakia limbata is the second acheilognathine host of A. coregoni. The occurrence of A. coregoni on the oily bitterling is unusual because this parasite usually infects various salmonids and ayu, Plecoglossus altivelis altivelis Temminck and Schlegel, 1846 (Plecoglossidae), in the upper and middle or lower-reaches of rivers, respectively, in central and western Japan. The individual of A. coregoni probably parasitized the oily bitterling after detachment from ayu near the collection locality.