The possibility of the use of electropherogrames of plasma proteins in polyacrylamide slab gel in the study of populations of marine cat-fishes was examined. A total of 176 samples of plasma from four species. Sciadeichtys luniscutis. Arius spixii, Genidens genidens and Netuma barbas was used. Inter-specific as well as intra-specific differences were found and the locus E of the plasma of all the four species showed a typical dialelic variation. The locus is recommended as one of alternatives which deserve due consideration in the planing of studies of populations of these species by means of biochemical methods.