The development of methods for measuring true stresses and strains in thermoplastics and of models for representing the results, makes it possible to predict polymer performance in a number of ways. Recently this method was used to study the stability of the tensile deformation of high-density polyethylene under adiabatic conditions. It was proposed that at high strain rates, thermomechanical softening would render the plastic deformation process unstable, promoting localised deformation and fracture. In this paper, the isothermal extension process measured at different temperatures is assumed to be stopped and then restarted after different draw ratios have been attained, as in the drawing of a fibre. In this way the effect of draw ratio on fibre tensile properties can then be predicted. It is shown that, with fast deformation under adiabatic conditions, the softening effect due to the increase in temperature exceeds the opposing influence of strain hardening so that the nominal stress is predicted to fall continuously with increased strain. This leads to a ductile fracture process, which, in a fibre, can generate mushroom shaped blobs of polymer at the broken ends. This effect has previously been reported by Hearle and co-workers. The applicability of the model to different type of fibre is considered. (C) 2003 Kluwer Academic Publishers.