Lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) of three related freshwater eels, Anguilla anguilla, A. rostrata, and A. japonica were examined by starch-gel electrophoresis using a citrate-aminopropylmorpholin buffer adjusted to pH 6.0. Five equal-spaced major bands were observed in LDH zymograms of the adult muscle exudation and the whole elver homogenate through the three species. The most anodal band was identified as the homotetramer of B subunits (B₄), and the most cathodal as the homotetramer of A subunit (A₄), based on the specificity of tissue distribution of these isozymes. These A₄, and B₄ bands of the Japanese eel were different from those of the European and American eels in electrophoretic mobility. In the MDH zymograms of muscle, four major bands were observed in all the three species. One of these, the activity of which was especially intense with the liver, was identified as A₂. The other two equal-spaced major bands of muscle were identified as AB and B₂, and the remaining most cathodal one as C₂. The European eel was distinguished from the American and Japanese eels by the A₂ band. The Japanese eel was also distinguished from the other species by the B₂ band. Although some variants were found in the LDH and MDH isozyme patterns of the respective species, their incidences were very low, and any alleles of these variants were not common among the three species.