The ϕ keratins extracted from individual turtles and snakes were compared. The extent and nature of the molecular differentiation associated with the production of structurally distinct epidermal tissues was determined. The electrophoretic comparisons, molecular weight, and chemical fractionation indicate that these tissues contain unique proportions of the constituent keratin monomers specific to each species. This pattern of differentiation is similar to that previously observed for avian scale, claw, and beak, and for mammalian horn and hoof. This suggests that several “scale-like” structures with distinctive chemical properties may be produced by a single individual without the synthesis of wholly unique proteins. The implications of these observations for the evolution of mammalian hair and avian feathers are discussed.