The heat-denatured proteins of canned fish obtained by treating the fish with cyanogen bromide yield polypeptides that are soluble in 6 M urea solution. These fragments of proteins, which have been cleaved selectively at the methionyl peptide bond, can be separated by electrophoresis in 6 M urea solution. The separation patterns for canned herring, tuna, plaice, salmon, lemon sole, sprat, haddock and mackerel are presented. The method extends the electrophoretic method of species identification to include canned fish.