Creep and superplasticity were investigated by testing a fine-grained extruded Mg-Zn-Zr magnesium alloy under a wide range of applied stress in the temperature range between 100 and 300 ℃. Grain boundary sliding became the dominating mechanism at 200 ℃, leading to a true superplastic behaviour at 300 ℃, where superplasticity was attained even under relatively high strain rates (5×10-3 s-1 ). By contrast, for lower temperatures, the straining process was controlled by dislocation climb. A comprehensive model, taking into account the simultaneous operation of the different mechanisms, was developed to describe the strain rate dependence on applied stress. Creep and superplasticity were investigated by testing a fine-grained extruded Mg-Zn-Zr magnesium alloy under a wide range of applied stress in the temperature range between 100 and 300 ° C. Grain boundary sliding became the dominating mechanism at 200 ° C, leading to a true superplastic behavior at 300 ℃, where superplasticity was attained even under relatively high strain rates (5 × 10-3 s-1). By contrast, for lower temperatures, the straining process was controlled by dislocation climb. A comprehensive model, taking into account the simultaneous operation of the different mechanisms, was developed to describe the strain rate dependence on applied stress.