The validity of the theory of linear viscoelasticity over the temperature range 20°-13°C has been tested by studying the tensile creep behavior of the amorphous, glassy polycarbonate based on bisphenol A. The stress threshold, beyond which the mechanical properties deviate from the predictions of linear theory by more than 3%, is found to decrease as the temperature approaches the vicinity of the glass transition temperature (149°C). Extension of the experimental time scale from 10 to 1000 sec likewise diminishes the stress level below which the linear theory is applicable. The onset of nonlinear behavior is marked by a clear increase in the isothermal, isochronous compliance with increasing stress. Such an increase in compliance with stress becomes gradually sharper at increasing stress levels until necking or failure of the specimen occurs. The analytical characterization of the transition from linear to nonlinear behavior, as well as certain molecular implications of such a transition, are discussed.