The large strain behavior of a glassy polymer was studied in terms of compression tests at various rates, while the local deformation was evaluated with a noncontact laser extensometer. The main features of yield and postyielding were described with a constitutive model from nonlinear viscoelasticity, combined with a kinematic formulation for the separation of total deformation into a viscoelastic and a plastic part, respectively. The concept of plastic shear transformations introduced elsewhere was used to develop a mathematical model for the rate of plastic deformation. The entire experimental true stress–strain curves (including strain hardening and strain softening) could, through this model, be identified in a self‐consistent manner. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2534–2542, 2001