The major, historically active San Jacinto and San Andreas fault systems pass through the San Bernardino Valley area of southern California. An array of six portable, high-gain seismographs was operated for five 2-week recording sessions during the summer of 1972 and winter and spring of 1973 in order to detail the microseismicity of the region. A crustal model for the Valley, modified after Gutenberg, was established using a 6-km reversed seismic refraction profile and a series of monitored quarry blasts. Fifty-five microearthquakes were used to establish a magnitude scale (1.5 to 3.3) based on coda lengths recorded by instruments peaked at 20 Hz. Forty-five hypocenters from the analysis of over 6,000 hr of low-noise records define two northeast trending lineations within the western portion of the Valley. A composite first-motion plot of 22 microearthquakes from these lineations indicates left-lateral strike-slip faulting. Fluctuations in microseismicity appear to reflect rapid changes in the stress patterns of southern California. Minor activity along the strike of the San Jacinto fault zone suggests a purely right-lateral strike-slip motion. Only minimal strain release was observed along the San Andreas fault zone.