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Land-based surveys were carried out to examine the behavioral changes of Indo-Pacific bottlenose dolphins Tursiops aduncus which reside year-round off Amakusa-Shimoshima Island, Japan, in response to dolphin-watching boats. A digital theodolite was used for positioning of dolphin groups. When the dolphins formed a compact group and repeated synchronized diving and surfacing, the presence of 1 boat led to an increase in time when they were under water (time from diving to subsequent surfacing) and in moving speed at the surface. The presence of 4-5 boats led to an increase in distance from the position of diving to that of subsequent surfacing and a decrease in time from surfacing to subsequent diving. Restrictions on the number of boats that can approach a dolphin group should be introduced.