A stable, homogeneous preparation of pyruvate kinase from white muscle of the American eel, Anguilla rostrata with a specific activity of 350 units/mg has been obtained. The enzyme has a pH optimum in the range 6.3-6.5 and requires Mg2+ and K+ for maximum activity. Eel muscle pyruvate kinase exhibits slight co-operativity in the binding of the substrate phosphoenol-pyruvate. It is activated by fructose-1,6-bisphosphate in a pH dependent manner and is inhibited by both alanine and phenylalanine. These properties are very similar to the properties of the mammalian M2 isozyme.