Frictional sliding experiments at confinnig pressures from 50 to 400 MPa were performed on thin layers of clay-rich fault gouges from locations along the San Andreas and Hayward faults in California as well as pure clays from other locations. Both dry and saturated, drained samples exhibited a strain hardening that increased systematically with increasing confining pressure for the each particular gouge. In addition, the amount of strain hardening was greater with progressively stronger gouges among the suite of different samples. Tests using various lubricating mediums at the sample ends and different jacketing materials all showed that these possible constraints on frictional sliding had no effect on the strain-hardening process. Therefore, the observed strain hardening was material property of the fault gouges. The presence of water lowered the strength, coefficient of friction, and amount of strain hardening of the samples. The coefficient of friction ranged from around 0.21 to 0.58 under saturated, drained conditions at 200-MPa confining pressure with the exception of pure montmorillonite. This sample had anomalously low strength and friction (μ~0.13) in relation to the other specimens. The strength of the gouges did not correlate well with mineral composition, grain size distribution, or location along the San Andreas fault; in particular, gouge samples from creeping and 'locked' sections of the fault showed no systematic differences in strength.