Impact shear tests of the Kolsky thin-wafer type were used to determine the effect of temperature and strain rate on the critical resolved shear stress for slip in aluminum single crystals at strain rates of 104 sec−1 and in the temperature range 20° to 500°K. The aluminum deformed in a viscous manner in that the flow stress was proportional to the plastic strain rate. The behavior was found to be temperature-dependent. The results are discussed in terms of dislocation damping models where the friction force acting on a dislocation results from, at cryogenic temperatures, electronic viscosity, and at higher temperatures, phonon viscosity. The theories predicted general agreement as to the magnitude of the observed damping but some discrepancy was found to exist between the observed and theoretical temperature dependence of the damping.