The Diamond Valley, California, earthquakes of September 1978 occurred near the southern termination of the north-striking, east-dipping Genoa Fault, a major normal fault exhibiting cumulative Holocene offsets of up to 10 meters along the eastern margin of the Carson Range. Master-event location of the 14 largest events (ML ≧ 3.0), using two close-in temporary stations for control, revealed a tight cluster 2 km in extent. P-wave first motions for the main shock (ML = 5.0) resolve a strike-slip mechanism with an east-west axis of minimum compressive stress. Faulting (right-lateral) was assigned to the southeast-striking plane on the basis of aftershock migration in that direction. This style of faulting partially accommodates the regional stress field in zones separating left-stepping normal faults of the Sierra Nevada-Great Basin boundary zone. Seismic moments, Wood-Anderson magnitudes, and stress drops were computed for aftershocks using close-in digital seismograms; stress drops were higher than those found by Douglas and Ryall (1972) for aftershocks of the 1954 Fairview Peak earthquake some 130 km to the east. One identifiable characteristic of this sequence is that the ratio of P-to S-wave spectral corner frequencies is considerably greater (2.5) than unity.