Mechanisms involved in cold hardiness of cocoons of the lumbricid earthworm Dendrobaena octaedra were elucidated by osmometric and calorimetric studies of water relations in cocoons exposed to subzero temperatures. Fully hydrated cocoons contained ca. 3 g water · g dry weight^-1; about 15% of this water (0.5 g·g dry weight^-1) was osmotically inactive or bound. The melting point of the cocoon fluids in fully hydrated cocoons was-0.20°C. Exposure to frozen surroundings initially resulted in supercooling of the cocoons dehydrated (as a result of the vapour pressure difference at a given temperature between supercooled water and ice) to an extent where the vapour pressure of water in the body fluids was in equilibrium with the surrounding ice. This resulted in a profound dehydration of the cocoons, even at mild freezing exposures, and a concomitant slight reduction in the amount of osmotically inactive water. At temperatures around-8°C, which cocoons readily survive, almost all (>97%) osmotically active water had been withdrawn from the cocoons. It is suggested that cold injuries in D. octaedra cocoons observed at still lower temperatures may be related to the degree of dehydration, and possibly to the loss of all osmotically active water. The study indicates that ice formation in the tissues is prevented by equilibrating the body fluid melting point with the exposure temperature. This winter survival mechanism does not conform with the freeze tolerance/freeze avoidance classification generally applied to cold-hardy poikilotherms. Implications of this cold hardiness mechanism for other semi-terrestrial invertebrates are discussed.