Pseudohermenias clausthaliana overwinters in the 3rd or 4th larval instar in a hollowed needle of Norway Spruce, roughly from October ist till April Ist. Within the geographical distribution of the species the larva is exposed to temperatures as low as - 30° C during this period. After emptying of the gut in September, the length of the larva is 5.3 mm ± 0.2, but hereafter it starts diminishing until the length is just 3.2 mm ± 0.2. During the first part of hibernation total weight decreases from 0.89 mg ± 0.06 to 0.68 mg ± 0.08; this drop is due to a loss of 29.5 % of the original water content. Dry-weight increases by as much as about 27.7 %, probably due to binding of about 7 % of the original "free" water content. Dry-weight, expressed as a percentage of the total weight, is I9.6 % ± 2.8 in the autumn and 33.3 % ± 2.5 in the winter. It has been suggested that the increase in dry-weight is caused by hydrolysis of certain organic compounds. Larvae exposed to I00 % R.H. and I8° C for 8 days without food at the beginning of March did not regain the original water content, while active larvae, feeding in the field in April, had regained the original high water content. Larvae at 85 % R.H. and I8° C for 8 days in March lost a little water but survived well, and they probably achieved an equilibrium with the surrounding air. Thus the critical equilibrium humidity seems to be less than 85 % R.H.