Measurements were made of the amounts of D,18O, and H2O+ in fault gouge collected over a depth of 400 m in the San Andreas fault of California. The amounts and isotopic compositions of the pore fluids, also analyzed, suggest that formation waters from adjacent Franciscan rocks have migrated into the gouge and mixed with local meteoric water. Thus the gouge is an open system permeable to fluid flow. This permeability has important implications concerning heat flow along the fault zone. Analyses of the fault gouge itself give information on the amounts, timing, and conditions of formation of the clay minerals. Stable-isotope analyses of materials from fault zones are good indicators of water-rock interactions that bear importantly on processes taking place in seismically active regions. ?? 1985 Birkha??user Verlag.