Indianmeal moth, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae), has been classified as one of the most cold-tolerant pests of stored grain. In this study, the supercooling point (SCP) of field-collected, cold-acclimated fifth instars was determined as an index of cold hardiness. In addition, mortality of laboratory-reared and field-collected, cold-acclimated fifth instars exposed to −10°C was measured to understand the ability of P. interpunctella to survive winter conditions. Finally, the overwintering mortality of this species in southern Minnesota was measured in grain bins filled with shelled corn. The SCP of field-collected, cold-acclimated fifth instars was approximately −24°C before their release in grain bins. Mortality of laboratory-reared fifth instars exposed to −10°C reached 100% after only 12 h, whereas the same percentage was achieved after 312 h for field-collected, cold-acclimated individuals. Overwintering mortality of P. interpunctella under field conditions reached 100% in all locations inside the grain bins; however, depth within the grain mass, location inside the grain bin, and the duration of exposure to outdoor conditions significantly affected the rate of mortality. A mathematical model for the overwintering mortality of P. interpunctella is proposed as a first step to forecast early season infestations under field conditions.