Determining the cold tolerance of mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae), is critical for assessing its long-term persistence and eruptive potential in its new habitat, as well as the risk of continued range expansion across Canada's boreal forest. We used supercooling points (SCPs) and mortality assessments with exposure to different temperatures to determine the cold tolerance of pupae. Mountain pine beetle pupae cold tolerance did not increase with chilling and there was little change in the lethal temperature regardless of treatment or sample time. SCPs were reflective of expected mortality due to freezing: the lethal temperature for 50% mortality was –19.3°C and the mean SCP was –18.7°C. However, significant mortality occurred over time at much warmer temperatures (0 and –9°C), indicating that this life stage suffers significant prefreeze mortality. On the basis of our results, it is unlikely that pupae would be able to successfully overwinter in most regions in Canada.This study is part of a larger project aimed at producing a comprehensive assessment of the cold tolerance of all life stages of the mountain pine beetle to feed population models, climatic suitability indices, and spread assessments.