Locations and magnitudes of a swarm of earthquakes in the hills east of San Francisco Bay have been estimated. The largest (ML = 4.3 ± 0.13 S.E.M.) event had an origin time of 09h38m07.5s ± 0.11s, January 8, 1977 UTC, a latitude of 37°54.31′ ± 0.69′N and longitude 122°10.97′ ± 0.50′W and a focal depth of 9.5 ± 0.8 km. The P fault-plane solution for earthquakes in the swarm was consistent with right-lateral slip on a plane with strike N28°W and dip 65°SW. No surface rupture was observed; but the mechanism is consistent with present regional deformation. Displacement seismograms recorded at Berkeley show a close similarity of wave form but the P/S amplitude ratios for the foreshocks are systematically greater than for the main shock and subsequent events. The calculated relative seismic energy suggests a very limited region of successive slip surfaces.