Impact shear tests have been used to study the dependence of shear stress with shear‐strain rate at strain rates ranging from 0.5×104to 2.2×104sec−1in pure Al, prestrained Al, and Al‐2 at. % Cu alloy. Within this strain‐rate range a linear relationship was found between shear‐stress and shear‐strain rates, indicating the presence of a viscous drag on dislocations. The dependence on temperature of this viscous damping was investigated between 10° and 293°K. The results are discussed in terms of theories of phonon viscosity and phonon scattering, and of electron viscosity at low temperatures.