A transitional-stage individual of the cymothoid iso- pod Nerocila phaiopleura Bleeker, 1857 was collected from the posterior body surface of an age-0 chub mackerel, Scomber japonicus Houttuyn, 1782, in coastal Pacific waters at Hitachinaka, Ibaraki Prefec – ture, central Japan. The transitional-stage individual of N. phaiopleura is briefly described. Ibaraki Prefecture is located at the northernmost distribution limit of N. phaiopleura in the western North Pacific Ocean, and the present collection of the species represents its sec- ond prefectural record. A light wound caused by N. phaiopleura was found at attachment site. Young chub mackerel are considered to serve as one of the major hosts for N. phaiopleura in Japanese coastal waters based on the fact that those fish have been reported to have the isopod infection at various coastal locations of the country.