Magnesium (Mg) alloys are currently the focus of a worldwide effort aimed at increasing their use in automotive components because of their low densities and high strength-to-weight ratios.  Because forming at elevated temperatures, e.g. 400 to 500 °C, provides excellent ductility in several wrought Mg alloys, hot forming is of great interest for producing these components.  Unfortunately, the availability of accurate material models for plasticity in wrought Mg alloys at elevated temperatures is severely limited.  The present study investigates a material constitutive model for high-temperature plasticity in a fine-grained Mg AZ31 sheet material, developed using data from uniaxial tension, and evaluates the accuracy and applicability of this constitutive model to finite-element-method simulations of forming under biaxial tension conditions.  Bulge forming experiments, which produce a nearly balanced-biaxial stress state, were conducted at 450 °C using four constant gas pressures.  Simulation results are compared with experimental results.  It is discovered that wrought Mg AZ31 behaves differently than wrought Al alloys previously investigated, e.g. fine-grained AA5083 sheet.  Recommendations on the use of experimental data for constitutive model construction and on applying constitutive models to forming simulations are made.