Caterpillars of the eastern North American moth Pyrrharctia isabella (Lepidoptera: Arctiidae) overwinter under plant debris where they are exposed to subfreezing temperatures. We measured supercooling points (scps), susceptibility to ice formation in body fluids at temperatures above the SCP following contact with external ice (inoculative freezing), freeze tolerance, cryoprotectant accumulation and metabolism of P. isabella caterpillars. They were collected between September 1996 and January 1997 and tested either immediately after collection (field acclimatization) or following 5 C acclimation in the laboratory. Hibernaculum temperatures were previously recorded between late November 1995 and late March 1996. In a dry laboratory environment, caterpillars had scps ranging between −5 to −10 C throughout the study. Moreover, caterpillars survived prolonged (7 days) supercooling at −5 C for 7 days and determinations of their scps, which caused brief internal freezing. We induced inoculative freezing of caterpillars by cooling them to −5 C on frozen substrate. Low postfreeze survival (14%) occurred during September 1996 following a 3-day freeze at −5 C but all caterpillars tolerated a 7-day freeze at −5 C in November (100%). Cold acclimation and seasonal conditioning increased hemolymph osmolality and glycerol content by 2× and 6–12×, respectively. Caterpillars showed partial compensation in O₂ consumption upon acclimation to 5 C vs. 25 C. Hibernaculum temperatures ranged from −7.5 C to 14.6 C during the winter of 1995–1996. Temperatures rarely dropped below the scp of caterpillars but the hibernaculum cooled below the melting point of their hemolymph (−1.6 C) for up to 2–4 days. Nevertheless, thaws were common and temperatures changed more than 5 C/day on 22 days. The present study demonstrates that the cold hardiness of Pyrrharctia isabella caterpillars allows them to endure subfreezing conditions beneath leaf litter in western Pennsylvania and that the metabolic capacities of these caterpillars allow them to exploit their environment during the late autumn and early winter.