An investigation on gastropod fauna was carried out on a tidal flat in the Nagura Estuary on Ishigaki Island, the Ryukyu Islands in 1989 and 1998 using similar methods. 470–480 quadrats covering ca. 1900 m² were surveyed during low tides from February to April in each year. Of the total 19 species recorded, the ranges of eight species had varied significantly between the two surveys, with six species expanding their range and two species contracting their range. Percentage in abundance of muddy-bottom species and tropical (<29°N) species increased significantly between the two years. Topography of the flat also changed: the mouth of the river was narrowed and the elevated sections of the tidal flat expanded. During the period from 1984 to 1998, the farmland development around the study site caused influxes of soil into the estuary and the sea-water temperature was rising. These results suggest that the topographical changes due to soil influx and the rising temperature affected the gastropod assemblage at the study site, by increasing the abundance of muddy-bottom species and tropical species. The methodology used in this study, i.e. surface observation at low tides, includes more than 95% of the gastropod fauna, demonstrating the usefulness of surface counts for the study of soft-bottom fauna.