During a faunal survey of the intertidal mud flat of Shinhama Lagoon on the northern coast of Tokyo Bay, parasitic bopyrid isopods, Ione cornuta Spence Bate, were found attached to the inner walls of the branchial chambers of ghost shrimps, Nihonotrypaea japonica (Ortmann). In this isopod, the females are parasites of N. japonica whereas dwarf males attach to conspecific females. To investigate the prevalence of the parasite I. cornuta, sampling of ghost shrimps was conducted every 2 to 4 weeks at the Shinhama Lagoon during the periods from August, 1994, to October, 1995, and January, 1998, to May, 1999. A total of 328 individuals (253 females, 62 males, and 13 of unknown sex) of N. japonica was collected, and 22 individuals among them (6.7%) were infected with I. cornuta. Among the infected shrimps, females accounted for 21 individuals, and only one male was infected. This skewed sex ratio is considered to simply reflect the female-biased sex ratio of this species in the field. In most cases, one individual of the parasite I. cornuta was attached within the left or the right branchial chamber of each host shrimp. No significant relationship was detected between the handedness of the major cheliped of the host shrimps and the attachment position of I. cornuta. A significant positive correlation was found between the size of the host shrimps and the size of mature females (i.e., those with brood chambers) of the parasite; however, the size of immature parasites was not correlated with the size of their host shrimp. In spite of the long sampling periods during this study, no ovigerous shrimp infected by I. cornuta was collected. This strongly suggests that infection by I. cornuta negatively affects the reproduction of the host female shrimps.