The membrane lipid composition of hepatopancreas tissue was analyzed from two phylogenetically related species of crayfish after thermal acclimation to either 5 or 20 degrees C. One species overwinters in a quiescent state (Orconectes propinquus) and the other remains active throughout the winter (Cambarus bartoni). Both species significantly decreased the level of saturates in the major phosphatides, phosphatidylcholine (PC) and phosphatidylethanolamine (PE; difference not significant in C. bartoni), and both winter-active and winter-quiescent animals tended to be enriched in n-3 acids in PC. Orconectes animals tended to be enriched in n - 3 acids in PC. Orconectes accumulated n - 6 acids in PE at low temperature, whereas Cambarus lost n - 6 acids in this lipid, a change that was offset by the accumulation of monoenes. Cardiolipin became more saturated with cold acclimation in both species. The fatty acid content of the triacylglycerol fraction showed little change with acclimation history, although O. propinquus did accumulate n - 3 acids in triglycerides at low temperature. In O. propinquus, head-group composition was not significantly altered by temperature, but in C. bartoni, cold temperature increased the relative amounts of PE, sphingomyelin, and phosphatidylinositol at the expense of PC. Adaptations to temperature in membrane composition of poikilotherms are related to the overwintering strategy of the animal.