The electrophoretic (starch gel) behaviour of non-specific esterases of the kidney of Selachii, Scylliorhinus stellaris and Torpedo marmorata has been investigated. In both species the enzyme shows a molecular heterogeneity. Differences in both the number and the activity of the isoenzymes are present in the species and in the two sexes. In Scylliorhinus stellaris, particularly, two isoenzymes migrating toward the cathode can be demostrated only in the male.