A build-up of reserve lipid, predominantly wax esters, occurred during the summer in the copepod Calanus hyperboreas, collected off an Arctic iceisland. This lipid storage was correlated with a phytoplankton bloom and was followed by a progressive decrease of lipid from 2.1 mg per individual in September to 0.4 mg in June. There was a rapid decrease in lipid utilization between October and December, followed by much slower decreases until June. Lowered respiration rates or the availability of different types or quantities of food in the winter and spring are possible explanations for the slower rate of lipid use. Laboratory starvation experiments for up to 90 days correlated with results from the field. Gas-liquid chromatographic studies of the lipids showed that both the alcohols and fatty acids of the wax esters were highly variable with season and depth, whereas the phospholipid fatty acids were not affected by changes in these parameters. Only summer samples had wax esters with a phytoplankton-like fatty acid composition, and upper water winter copepods had wax esters with little polyunsaturation. The deep-water winter copepods had a very different wax-ester composition from the upper water samples, with a predominance of hexadecanol (all other copepod samples had 20:] and 22:11 as the principal alcohols) and a high content of polyunsaturated acids. Deep-water C. hyperboreas may differ in food habits and life history from those in the upper water community.