1. Pytho deplanatus (Coleoptera: Pythidae (Salpingidae)) is an alpine species that overwinters as a larva under the bark of fallen spruce in the Rockey Mountains. It belongs to a small group of insects which not only is tolerant of freezing but also develops the capacity to supercool to low sub-zero temperatures (-54 C), indeed in this case the lowest ever recorded for this category. 2. The freezing tolerance profile of P. deplanatus is similar to that of many other insects in that glycerol is accumulated to moderately high levels during low temperature acclimation, while water content and glycogen concentrations are considerably reduced. Blood sugar levels, mainly trehalose, also accumulate during this period. 3. The most feasible explanation for the co-existence of freezing tolerance and low supercooling points is that P. deplanatus is physiologically incapable of synthesizing haemolymph nucleating agents and that nucleators in the gut and other fluid compartments are absent in the overwintering larvae. The adaptive advantage of this strategy is that larvae would normally overwinter in a supercooled state, but if freezing did occur (e.g. by inoculation from contact water in the hibernaculum) then the antifreeze function of gycerol and blood sugars would assume a cryoprotective role. 4. The freezing tolerance mechanisms of P. deplanatus larvae are compared and contrasted with those of (a) the congeneric P. americanus which also overwinters under the bark of fallen trees but has both an alpine and arctic distribution, and (b) Xylophaqus sp (Diptera Xylophagidae) which has a different phylogenetic background but occupies a similar niche in the Arctic.