A simple analytical model for the adiabatic high strain rate extension of synthetic textile fibers is presented. The model suggests that, for fibers with particular thermo-mechanical and constitutive properties, initial nominally uniform strain distributions along the fiber will tend to become non-uniform, with localization of axial strain into a thermally softened region. To assess the usefulness of the model in predicting and interpreting fiber behavior, a commercial nylon filament is investigated experimentally. Nylon filaments are extended to break at a low, isothermal strain rate (0.0015 s(-1)) and at a high, adiabatic strain rate (70 s(-1)). A dimensionless strain localization parameter (SLP), used to characterize the nylon filament in the framework of the model, predicts strain localization to occur during extension at the 70 s(-1) strain rate. Experimental load-extension curves exhibit a sharply reduced elongation-to-break at the high strain rate, consistent with the predicted occurrence of localized, versus uniform, straining. In addition, the transition from homogeneous to localized straining appears to occur at elongations that correspond with the SLP attaining a critical value for onset of localization. (C) 2003 Kluwer Academic Publishers.