Four specimens of crustacean parasite were collected from the caudal fin and caudal peduncle of a bastard halibut Paralichthys olivaceus (Temminck and Schlegel) cultured in Saiki Bay, the western Bungo Channel (western North Pacific), off Shirahama, Oh-nyu Island, Saiki, Oita Prefecture, Japan in June 1992. The specimens were lost but one of them was identified as the argulid branchiuran Argulus matuii Sikama, 1938, based on its morphological characters recorded on a photograph of the whole body. This finding represents the first record of A. matuii from farmed marine fishes. P. olivaceus is a new host for A. matuii. A total of 8,165 specimens belonging to 57 species of marine fishes cultured in coastal waters of Oita Prefecture were examined for parasitic diseases for 18 years from 1980-1997, but only three cases of argulid infection, including the present case, were found, indicating that argulids are very rare parasites in finfish mariculture in this prefecture.