Sea bass (Dicentrarchus labrax L.) white muscle pyruvate kinase activity derives from only one enzymatic form. Chromatographic and electrophoretic analysis revealed a single activity peak. This enzyme shows phosphoenolpyruvate-positive cooperativity at 5, 10, 15, 22 and 27°C and the temperature does not change significantly its S_0.5 and n_H values. It is clearly activated by fructose-1,6-bisphosphate and this effect is modified by temperature which could reflect a protector effect on the enzyme. A decrease in assay pH from 7.4 (maximal) to 6.8 (near physiological value) produces an increase in the phosphoenolpyruvate-enzyme affinity. Fructose-2,6-bisphosphate behaves as activator and creatine phosphate displays an inhibitor effect which is removed by both bisphosphorilated hexoses. Further, fructose-1, 6-bisphosphate and fructose-2, 6-bisphosphate together produce a decrease in S_0,5. An active glycolytic flux in sea bass white muscle for anaerobic energy releasing is apparent.