Temora longicornis, a calanoid copepod with low energy reserves and high metabolic requirements, is a key species in the North Sea zooplankton community. Since the North Sea is characterised by high seasonal variation in food quality, the digestive system of T. longicornis must respond fast to use different food sources efficiently. Lipids play a major role in studies of copepod feeding, because (1) they are an important energy source, and (2) different diets can be traced by specific dietary fatty acids. Information, however, on the capacity of the lipid metabolism to adapt to changing food is scarce. Therefore, we have studied the fatty acid composition and lipolytic enzyme patterns of T. longicornis in relation to food supply. Adult females were incubated for three days, feeding either upon the diatom Thalassiosira weissflogii or upon the heterotrophic dinoflagellate Oxyrrhis marina. The enzyme patterns of single females were identified by SDS-PAGE using a fluorescent substrate for lipases/esterases at the start and the end of the experiments. The fatty acid compositions were analysed by gaschromatography. In females fed with T. weissflogii the portion of the fatty acids 16:2(n-4) and 16:3(n-4) increased by a factor of 2.5 and 6.3, respectively. The portion of 22:6(n-3) doubled in individuals fed with O. marina. This shows that the dietary fatty acids have been digested and incorporated in the copepods lipids within three days. In both groups, new enzyme bands (30 to 100 kDa) appeared, indicating a high adaptive capacity of the lipolytic enzyme complex to different diets.