An insect's cold hardiness affects its potential to overwinter and outbreak in different geographic regions. In this study, we characterized the response of Helicoverpa zea (Boddie) pupae to low temperatures by using controlled laboratory measurements of supercooling point (SCP), lower lethal temperature (LT(50)), and lower lethal time (LLTime). The impact of diapause, acclimation, and sex on the cold hardiness of the pupae also were evaluated. Sex did not significantly affect the SCP, LT(50), or LLTime. However, the mean SCP of diapausing pupae (-19.3°C) was significantly lower than nondiapausing pupae (-16.4°C). Acclimation of nondiapausing pupae to constant temperatures from 10 to 20°C before supercooling also produced a significantly lower SCP than nondiapausing pupae held at 25°C. The LT(50)s of nondiapausing and diapausing were not significantly different, but confirmed that H. zea pupae are chill-intolerant because these lethal temperatures are warmer than the corresponding mean SCPs. Diapausing pupae survived longer than nondiapausing pupae at the same, constant, cold temperatures, a finding consistent with the SCP results. Both of these results suggest enhanced cold hardiness in diapausing pupae. When laboratory results were compared with field temperatures and observed distributions of H. zea in the contiguous United States, the laboratory results corroborated what is currently perceived to be the northern overwintering limit of H. zea; approximately the 40(th) parallel. Moreover, our research showed that areas north of this limit are lethal to overwintering pupae not because of low temperature extremes, but rather the length of time spent at near-zero temperatures.