The Northern krill, Meganyctiphanes norvegica (Crustacea, Euphausiacea) is widely distributed in the northern and northeastern parts of the Atlantic Ocean where it faces rapid variations in water temperatures and food. We studied the physiological potential of krill to compensate for environmentally induced metabolic changes. Two isoforms of the glycolytic key enzyme pyruvate kinase (PKI and PKII, EC 2.7.1.40) were partly purified from M. norvegica by anion exchange chromatography. Specific activities and catalytic properties of each isoform were determined in whole body extracts as well as in selected organs and tissues of males and females. Both PK-isoenzymes differed slightly in their temperature profiles, their activation energy and their molecular weights. PKI showed a high affinity for the substrate PEP and was not affected by fructose-1.6-bisphosphate (FBP). In contrast, PKII showed low affinity for PEP but was strongly activated by FBP, up to 40-fold. The specific PK-activity of whole organisms was lower in females (44.9 ± 4.8 U•g ww- 1) than in males (61.3 ± 7.7 U•g ww- 1). In females PK II represented 20% of the total PK-activity while it was only 10% in males. Highest PK activities were present in the hearts, the eyes, pleopods and in the thoracopods. In the stomachs and the midgut glands PK activities were low. Almost all organs contained PKI and PKII. However, PKI prevailed in the abdomens, the pleopods, the thoracopods, and in the thoracic muscles. PKII dominated in the eyes, the midgut glands and in the ovaries. Experiments showed that the tissue concentrations of FBP increased with food uptake and temperature. The expression of two PK-isoforms with different kinetic properties and the mediation of substrate affinity by FPB is a powerful tool to immediately regulate glycolytic energy flows in different organs. The krill is capable of adjusting energy consumption to changes in nutritional conditions as well as variations of environmental temperatures. © 2005 Elsevier B.V. All rights reserved.