The species that belong to the bivalve genus Geloina occur in mangrove forests in the western Pacific and Indian Ocean. They are representative species of bivalves that inhabit the mangrove forests. In Japan, G. expansa and G. yaeyamensis are classified as vulnerable species in the Red Data Book. It is necessary for them that a method of conservation is founded at the earliest possible moment. However the detailed studies about their biology and surroundings are few. In order to clarify the suitability of the environment for them, we investigated in the Urauchi River that is the longest river in Okinawa in September and December 2011. In the mangrove forests along the river, 20quadrats (1m²) were established. The occurrences of these species were recorded. The characteristics of the sediment were explained with temperature, ORP, pH, salinity and grain size distribution. Besides the occurrence and plant species were surveyed in the larger quadrats (100m²) that were set around each quadrat. These species tended to appear in the areas where Bruguiera Gymnorhiza and Pandanus odorattisimus were dominant species. These two plants dominate in the area that is not easy relatively to be influenced by a tidal current. On the other hand, in some quadrats that had occurrence, salinity was extremely high. It was inferred that Geloina spp. tended to be affected by a tidal current physically.