Water level, temperature, salinity, electric conductivity, and pH have been measured periodically for several years at three water wells located along a 17-km segment of the San Andreas fault between San Jaun Bautista and Cienega Winery in central California. Water samples were collected at the same time for subsequent chemical analyses in the laboratory. Some sudden large changes in salinity and conductivity were recorded in early March 1980 at the two wells near San Juan Bautista. These changes coincided approximately with the beginning of an episode of increased local seismicity, including a magnitude 4.8 earthquake on April 13. Analyses of water samples revealed corresponding changes in ion concentrations, especially of Na+, Ca++, Mg++, SO-4-, HCO-3, F-, and Cl-. The observed changes may be the result of mixing of waters from different aquifers through cracks developed in the water barriers by a possible crustal strain episode that may have occurred in the study area.