Cold hardiness in the Arctic Collembola Megaphorura arctica (Tullberg), formerly Onychiurus arcticus, has been the subject of extensive studies over the last decade. This species employs an unusual strategy known as cryoprotective dehydration to survive winter temperatures as low as -25 degrees C. To expand knowledge of cryoprotective dehydration in M. arctica, the present study investigates how a reduction in ambient temperature affects the fatty acid composition of the total body lipid content along with polar (mainly membrane phospholipids) and nonpolar (mainly triacylglycerols) lipids. Most ectothermic animals compensate for changes in fluidity by regulating fatty acid composition, a process often described as homeoviscous adaptation. In M. arctica, changes in the fatty acid composition of total body lipid content during cold treatment are only moderate, with no clear pattern emerging. However, the levels of unsaturated fatty acids in the polar lipids increase with cold exposure, largely attributable to 16 : 1(n - 7), 18 : 1(n - 9), 18 : 3(n - 6) and 18 : 3(n - 3), whereas unsaturated fatty acid levels in the nonpolar lipids correspondingly decrease. These results suggest a reallocation of fatty acids between the two lipid pools as a response to a temperature reduction of 6 degrees C. Because of hypometabolism, a characteristic of cold adaptation, such a mechanism could be less energy demanding than de novo synthesis of fatty acids and may comprise part of an adaptive homeostatic response.