Winter mortality is expected to be a key factor determining the ability of mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae), to expand its range in Canada. We determined the mortality rate and supercooling points of eggs from the beetle's historic range in southern British Columbia as well as the recently expanded range in north-central Alberta and tested if eggs require an extended period of chilling to reach their maximum cold tolerance. We found no effect of population source or acclimation time on egg cold tolerance. Although 50% of eggs can survive brief exposure to –20.5 °C (LT₅₀), storage at 0.3 °C and –7.5 °C for 59 d resulted in 50% and 100% mortality, respectively. Our results indicate that eggs suffer significant prefreeze mortality and are not well-adapted to overwintering: eggs are unlikely to survive winter throughout much of the beetle's range. Our results provide information that can be used to help model the climatic suitability of mountain pine beetle, including how changes in seasonality associated with new or changing climates may affect winter survival. In addition to lower lethal temperatures, it is critical that the duration of exposure to sublethal cold temperatures are considered in a comprehensive index of cold tolerance and incorporated into survival and population models.