The seasonal variations in glycerol content and supercooling temperatures in the eggs of the forest tent caterpillar, Malacosoma disstria Hübn., were investigated. Glycerol accumulated in the autumn and early winter and reached a peak in January when 35·2 per cent (dry weight basis) was recovered from eggs and decreased to <1·0 per cent during the post-diapause stage in spring just before hatching. Temperatures between 10 and 32·2°C caused a rapid decrease in glycerol during post-diapause. At 5°C the glycerol concentration decreased more slowly and the eggs did not hatch. Supercooling temperatures of −29·5°C were recorded for summer eggs, the lowest supercooling temperatures occurred from November to April, when they ranged from −32·6 to −40·8°C, and did not increase until a few days before hatching after exposure to warm spring temperatures. The concentration of glycerol is not completely correlated with cold-hardiness; accordingly other factors additional to glycerol are presumably involved. Approximately 80 per cent of newly hatched unfed larvae survived for 10 days at −1·0 to 15°C, but below and above this range survival was low.