The internal friction of Mg was studied at 4 kc/s and 40 kc/s as a function of temperature (4.2-300°K), plastic deformation, crystalline orientation, heat treatment and impurity content. In pre-strained polycrystalline samples a peak was observed at 20°K which annealed out readily at 300°K, and a very broad peak between 40 and 240°K which showed a complex dependence on deformation and annealing. In deformed single crystals only the broad peak was found. The variation with orientation indicates that the peak is caused by dislocation motion in the (0001 ) slip system. The annealing behavior points to a possible dissociation of non-basal dislocations into basal and non-basal components. Addition of 1% Li suppressed both peaks, while Ni and Fe (which form precipitates) enhanced them. © 1967.