Five ovigerous females and five males of cymothoid isopod were collected from the branchial cavity of five individuals of Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846), in Hiroshima Bay, the Seto Inland Sea, Hiroshima Prefecture, Japan. They were found in female-male pairs. The isopod re- sembled Mothocya parvostis Bruce, 1986 and Motho- cya sajori Bruce, 1986, but the body length (BL) of the five ovigerous females ranged from 17.0 to 19.4 (mean, 18.4) mm, which do not correspond to the BL ranges of the ovigerous females of M. parvostis (11.0– 15.0 mm) and M. sajori (20.5–27.5 mm) reported in the original descriptions of these species. Thus, the isopod collected cannot be identified as either M. par- vostis or M. sajori and is herein reported as Mothocya sp. It is highly probable that M. parvostis and M. sajori are indistinguishable from each other and regarded as conspecific. A taxonomic work of these two species is needed.

Mothocya parvostis or Mothocya sajori ?: cymothoid (Isopoda) parasitic on Japanese halfbeak, Hyporhamphus sajori, in Hiroshima Bay, the Seto Inland Sea, Japan (Kazuya Nagasawa)