When magnesium single crystals of 99.99% purity were prestrained at a low temperature (e.g. −196°C) and subsequently deformed at a high temperature (e.g. room temperature), both the yield drop in the stress-strain curve and the propagation of Lüders bands were observed. The effects of the prestraining and retesting temperatures as well as low temperature annealing treatments were studied. It was shown that the sharp yielding was not due to the strain aging induced by impurity diffusion but due to the work softening. Microcopic observations showed that the surface markings of the work softened region consisted of broad, concentrated slip bands whereas the region which was not work softened had sharp slip markings. No cross slip was observed. It is suggested that the work hardening at low temperatures is due to the immobile character of the jogs created during deformation. Annihilation of the jogs through their movement along dislocations under applied stress with the aid of thermal fluctuation at high temperatures is proposed as a possible mechanism to explain the work softening in magnesium.