Activities and characteristics of two metabolic key enzymes, citrate synthase (CS) and pyruvate kinase(PK), were studied in the Northern krill, Meganyctiphanes norvegica, with respect to adaptive properties underdifferent thermal conditions. Krill were sampled during late winter/spring and summer from the constantly warmLigurian Sea (12-13°C below the thermocline), the colder but also comparatively constant Clyde Sea (7-8°C), andthe variable Kattegat (2-16°C). Both enzymes showed distinct tissue- and organ-specific activities, which werehighest in the pleopods - the principal locomotive organs. The fourth and fifth abdominal segments, however, wereused for routine investigation due to lowest variability. Specific activity of CS and PK did not differ between seasonsin krill from the Kattegat or the Clyde Sea. In the Ligurian Sea, in contrast, specific CS activities were significantlylower during summer. Analysis of individual data illustrated a decrease of CS activity with size and an increase of PKactivity with size. Taking these allometric effects into account, as emphasized by calculating the ratio between bothenzymes, variation of CS and PK activities turned out to be solely dependent on body size, which differed betweenlocations and seasons. Ligurian krill from the summer, however, were unique in that they showed a lower CS/PK ratiothan would be predicted by the scaling effect. Thermal characteristics of each enzyme were similar between locationsand seasons. During the winter, in Kattegat and Clyde Sea krill, Km values (Michaelis-Menten constant) of CStowards acetyl-coenzyme A exhibited an almost constant level over the experimental temperature range of 4-16°C.During summer, however, Km values were lower at 8°C in the Clyde Sea and at 12°C in the Kattegat. In Liguriankrill from the summer, Km values were consistently lower than those of winter krill over the entire experimentaltemperature range. In conclusion, Kattegat and Clyde Sea krill show only minor adaptations to their respectivethermal environments in terms of CS and PK characteristics. Ligurian krill, in contrast, exhibited decreased specificCS activity during summer, which might be compensated by elevated enzyme-substrate affinity as indicated by lowerKm values. Since temperature was constant during both seasons, this effect cannot be explained as a reaction tothermal conditions. Consequently, oligotrophic conditions in the Ligurian Sea during summer may entail a reduction inthe somatic performance of krill, which is reflected by lower CS activity.