Deep-sea hydrothermal vent activities were studied done by digital OBS/Hs(Ocean Bottom Seismometer / Hydrophone) at three hydrothermal vent areas, the Mariana Trough, the Okinawa Trough, and the TAG area in the Mid-Atlantic Ridge. The OBS/H has three-component seismometers and one hydrophone. The hydrophone has a response from dc to 50 Hz including recording characteristics. The seismometers are made up of one vertical and two horizontal components with a natural frequency of 2 Hz and a response to particle velocity of ground motion. At each location, OBS/Hs were placed at a 1-m distance from nearby fissures or a hydrothermal mound by using submersibles. During a 1-week observation at each location, high-amplitude pulse-like signals were observed only by the hydrophone, but not by seismometers, at all locations. The amplitude of pulses on the hydrophone is extremely large. The hydrophone also recorded numerous natural earthquakes with amplitudes similar to those on seismometers. The appearances of pulses are highly coherent with tide variation. Based on the above results, it is concluded that the tide effect possibly controls the deep-sea hydrothermal vent activities observed as a pressure change.