Abstract Two American and eight European collections of silver eels were examined electrophoretically for variations in their liver esterases. Considering the results together with those obtained previously, it was confirmed that there is large variation in phenotype frequencies within both the eastern and the western Atlantic populations. Such quantitative data are, therefore, unlikely to resolve the question whether the two populations are produced from one or from two gene pools. The concept of separate gene pools, i.e., the classical two-species theory, is supported by certain differences in the esterase patterns which are not quantitative but consist in the complete absence from the one but presence in the other population of particular electrophoretic bands, or particular molecular properties reflected in consistent dissimilarities of electrophoretic mobility. A system of esterases is described which is shared by silver eel liver and whole homogenate of elvers. This system could be used in comparing phenotype frequencies in elvers and adult eels, and thus detect changes in the freshwater stages caused by differential selection.