Knowledge of the magnitude of the differential stress during the formation of mylonitic rocks provides constraints on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low-angle detachment faults. We have analyzed the differential flow stress during the mylonitization of quartzone rocks in the Ruby Mountains, Nevada, using grain-size piezometers and kinetic laws for grain growth. Quartzites from mylonitic shear zones in Lamoille Canyon and Secret Creek gorge have grain sizes of 91--151 μm and 42--64 μm, respectively. The peak temperature during mylonitization was 630¿¿50 ¿C, and analysis of grain-growth kinetics indicates that mylonitization continued during cooling to temperatures ≤450 ¿C. Quartz grain-size piezometers suggest that the mylonitization occurred under differential stresses (&sgr;1--&sgr;3) of 38--64 MPa, or maximum shear stresses of 19--32 MPa. Extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10-10 and 10-13 s-1; arguments presented in the paper suggest that the likely range of strain rates is 10-11 to 10-12 s-1. These strain rates are compatible with displacement rates of the order of 23 mm yr-1 along a 1.5-km-thick simple shear zone. Such a shear zone dipping 15¿ would produce an uplift rate of 5.8 km m.y.-1 and a horizontal extension rate of 22 km m.y.-1. This uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1.5-km-thick simple shear zone dipping 15¿ in 2.6 m.y. ¿ American Geophysical Union 1990