Three (two ovigerous and one non-ovigerous) fe- males of cymothoid isopod were accidentally collected in June 1998 from the branchial cavity of Japanese halfbeak, Hyporhamphus sajori (Temminck and Schle- gel, 1846), in the coastal Sea of Japan off Mashike, Hokkaido Island, northern Japan. The females are briefly described. They resemble Mothocya parvostis Bruce, 1986 and Mothocya sajori Bruce, 1986, but the two ovigerous females are 16.4 and 16.9 mm in body length (BL), both of which ate between BLs reported for M. parvostis (11.0–15.0 mm) from Japanese half- beak and two other fishes and M. sajori (20.5–27.5 mm) from Japanese halfbeak in the original descrip- tions of these species. Thus, the specimens collected are herein reported as Mothocya sp. The results imply that BL is not a reliable key to differentiate both spe – cies from each other, and it is desirable to examine variations in BL and other taxonomic characters using many specimens of Mothocya from Japanese halfbeak. A brief review of previous records of Mothocya spp. from Pacific saury, Cololabis saira (Brevoort, 1856), and an unknown host from waters around Hokkaido Island is also given.