One of the most challenging activities for magnesium industry is to increase productivity by introducing cost-effective processes. While die-cast magnesium alloys are in worldwide use, even in the demanding sector of automotive industry, where cost is the major asset, wrought magnesium alloys still not widely available. The aim of the present study was to evaluate the high temperature response by torsion testing of a ZM21 alloy, to calculate the constitutive equations to be used in FEM simulation of the extrusion process. The ZM21 alloy that was considered in the present study was Direct-Chill casted by Alubin Ltd., Israel. Torsion tests were carried out in air on a computer-controlled torsion machine, under strain rates ranging from 10-2 to 5 s-1 and temperatures from 200 to 400°C. The equivalent peak flow stress was related to temperature and strain rate by means of the conventional power-law and by the sinh equation. The microstructure of the alloy, even at 300°C, appeared largely unrecrystallized, with elongated grains; also at 400°C, the structure is more equiaxed, but elongated structures still appears.