This paper discussed Coulomb failure criteria for brittle deformation of intact rock and fault gouge. Data are presented from laboratory experiments designed to identify the critical gouge layer thickness required to effect a transition from the standard Coulomb criterion to a modified failure law (referred to as Coulomb plasticity) appropriate for simple shear of a gouge layer. Experiments were carried out using tension fractures and quartz power to simulate granular fault gouge. Fractures sheared without gouge obey the standard Coulomb law. A 0.6 mm-thick gouge layer was required to effect the transition to Coulomb plasticity. I test and reject the hypothesis that fault zone strength and apparent coefficient of internal friction can be predicted from fracture of intact rock simply by accounting for differences in the failure laws and without considering variations in the Coulomb parameters. The data presented indicate that the stress state required for Coulomb plasticity is not developed within very thin gouge layers. This work implies that brittle fault zones have lower friction than predictions based on the strength of intact rock. However, the magnitude of this weakening effect is small (for example, a coefficient of sliding friction of 0.75 would be reduced to 0.6) and thus it is not an independent explanation of the apparent weakness of mature faults. ¿ American Geophysical Union 1995