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Abstract—Isozyme genotypes of 400 glass eels recruiting to 4 localities along the east Asian coast, stretching from Taiwan to the Yalu River of northeastern China, were studied using starch gel electrophoresis. Geographic cline was found to exist in two loci: NADP‐isocitrate dehygenase‐1 and 6‐phosphogluconate dehydrogenase. Frequencies of the most common allele of these two loci increased from south to north. In the latitudinal range of 25°N to 40°N, the magnitude of difference of IDH100 and PGD100 was 13% and 9% respectively. However, deviations from Hardy‐Weinberg equilibrium were found to be insignificant in both loci at three of the four localities. The cline was, therefore, unlikely to have resulted from selection. Migration time‐lag from different parts of the continent to the spawning ground in the western Pacific was suggested to be a possible reason for the formation of the cline.