An OBSH (ocean bottom seismometer with hydrophone) was deployed just 1 m from a hot thermal vent on the summit region of a seamount located 18 km east of the south Mariana Trough axis to record continuous data on hydrothermal activity. Although the seismicity beneath the seamount was very weak, numerous pulse shape (pressure) events showing sudden pressure decrease were observed on the hydrophone channel. The characteristic period of the pressure events varied from 20 seconds to 1 minute. After the submersible Shinkai 6500 relocated the OBSH 10 m south of the vent, where no thermal vents existed, the number and amplitude of pressure events drastically decreased. This suggests that pressure events were generated by the hot water upwelling from the thermal vent. The pressure events appear to be related to the ocean tide. This experiment indicates that observations using OBSHs are an effective means of monitoring hydrothermal activity. ¿ American Geophysical Union 1995