We have made the first multi-year deployments of a small number of ocean bottom seismometers on a mid-ocean ridge. We located our instruments in and around a vigorous hydrothermal vent field on the Endeavor Segment of the Juan de Fuca Ridge. The deployments have revealed a consistent pattern of activity comprising several types of events. Most abundant are swarms of extremely small microearthquakes that we suggest are associated with cracking of the oceanic crust. We also observe local tectonic earthquakes that occur about four times per day. Preliminary hypocentral locations place these tectonic events away from the ridge axis and outside the hydrothermal field. The seismicity on the Endeavor Segment is remarkably different from the southern Juan de Fuca Ridge where little, if any, tectonic activity is occurring. We speculate that the Endeavor and southern Juan de Fuca Ridge are in two different phases of their long-term evolution. The latter has only rare activity that is only beginning to focus hydrothermal circulation along faults. In contrast, the Endeavor Segment is in an active tectonic phase; and the faults have had time to develop robust hydrothermal circulation, resulting in large hydrothermal vent deposits. ¿ American Geophysical Union 1993