THE Ichneumonidae (Hymenoptera) are a family of parasitic wasps (15,000 species in the world1) which deposit their eggs in larvae or pupae of other insects. Twelve genera can overwinter as free adults, and individuals of these genera may be found hibernating in rotten forest logs, between rocks and under moss carpets. It seems likely that species that survive the winter possess some physiological mechanism absent in non-hibernating species. Glycerol and sorbitol accumulate in a limited number of unrelated diapausing and non-diapausing insects subjected to temperatures resembling overwintering conditions2. This study was undertaken to see if (a) the overwintering ability of a group of closely related species is manifested by a common observable phenomenon, namely polyol accumulation, and (b) this phenomenon is restricted to those species which hibernate.