As the Okinawa Trough is a back-arc basin in early spreading, modern submarine hydrothermal activity and minerallization have many characteristics which have aroused wide attention. Up to now, three well-known hydrothermal venting areas are all located in the middle part of the trough. During two cruise investigations to map and sample the seafloor, numbers ofCalyptogena sp. shells were dredged at two sites in the northern trough with comparatively thicker crust and numerous submarine volcanoes. Based on the fact thatCalyptogena sp. is only observed around the hydrothermal vents and lives on hydrothermal activities, it is predicted that there is the possibility of modern hydrothermal activities in the northern part of the trough. In this note, the shell is carefully characterized and the sample locations with possible hydrothermal activity are given. It is pointed out that the research of biogenic fossils to trace hydrothermal activity changes in venting time, strength fluctuations, evolution in chemical compositions and so on should be stressed in the future in addition to the study of the ecological characteristics of hydrothermal organisms.