The esterase enzymes of the tissues of Atlantic herring (Clupea harengus harengus) were analyzed by starch gel electrophoresis. Four sets of esterase bands were distinguished by their electrophoretic mobility, their relative activity with the two substrates, alpha-naphthyl acetate and alpha-naphthyl butyrate, and their relative concentrations in plasma, liver, and heart tissues. All of the esterases were inhibited by 10−4M solutions of dichlorvos, an organophosphate inhibitor, but none was inhibited by 10−4M eserine sulfate or by 10−4M EDTA. Polymorphism was noted in all four sets of esterases. Evidence for the genetic control of the fastest migrating set was obtained from population genetic analyses. In this set of esterases, five distinct bands occurred either singly or in pairs. The observed distribution of the three most common bands fits the hypothesis that they are controlled by a set of autosomal alleles. The two rarest bands occurred only in the heterozygous state, as would be expected. Differences in frequencies of two of the genes were detected between herring taken in Western Maine waters and on Georges Bank.