A new seismographic array with a band width of 500 Hz per channel and a dynamic range of 96 dB was developed for detecting natural events on glaciers. It was first deployed on ice stream B during the 1985–86 austral summer. The network consists of nine solar-powered seismographs, each monitoring three components of ground motion. Each of the seismographs is connected by up to 4 km of fiber-optic cable to a central node where seismic events are both detected and recorded. During 85 h of passive seismic monitoring on ice stream B, 25 microearthquakes were observed. Sixteen of these events were associated with shallow crevassing, mostly near the margins, although not within the zones of extreme shearing that bound the ice streams. Nine microearthquakes were associated with low-angle thrusting near the base of the ice stream. The principal initial result of these passive seismic studies is the demonstration that virtually none of the energy dissipated beneath ice stream B takes place through brittle fracture near the base. Nevertheless, fracture associated with microearthquakes may play a significant role in sub-glacial erosion.