Experimental data on the deformation of polyethylene terephthalate over a range of temperatures below its melting point are presented. The observed behavior is of the type usually considered as “plastic”; in spite of this, it is described by a modification of a constitutive description initially proposed for molten polymers by Tanner and Simmons. The constitutive description proposed here correctly portrays the observed behavior without reference to classical plasticity theory, and is of a type which allows in principle to predict the behavior of the material for every conceivable deformation history.