High gain portable seismographs have been operated at 22 sites along the San Miguel fault zone, Baja California, Mexico. We report results in terms of normalized rates of microearthquakes per day. Very high rates, exceeding 100 events/day, were observed near the southeast end of the San Miguel fault. Sixteen of the 22 sites recorded rates exceeding 20 events/day. The overall micro-seismicity is higher than along most of the San Andreas fault in Southern California. At two sites, arrays were operated to obtain accurate locations. Locations near the southeast end of the San Miguel fault were very near the fault trace and at depths of 8 to 14 km. Relatively high microearthquake rates were observed at a site 70 km from Tijuana, suggesting that the northwestern San Miguel fault zone is active and might pose a significant seismic harzard to the cities of Tijuana and San Diego.