Mortality of diapausing and non-diapausing pupae of Heliothis virescens (Fabricius) and H. zea (Boddie) at low temperatures was assessed in the laboratory. Comparison of the exposure time necessary to induce 50% mortality (LT50) indicated that survival of diapausing pupae was significantly higher than that of non-diapausing pupae. Similar comparisons showed no consistent significant differences in mortality of pupae from different geographic locations within Texas or in pupae preconditioned in several different ways. One exception was the response of pupae of H. virescens preconditioned to varying temperatures. This type of preconditioning appeared to increase the cold-hardiness of pupae at subfreezing temperatures. Contact moisture significantly decreased LT50 values of pupae of both species at sub-zero temperatures. Predictive models for low temperature mortality were developed for dry, diapausing pupae.