The elephant, black and white rhinoceros and the hippopotamus inhabit similar country, feed off land vegetation and have superficially the same type of almost hairless skin. The hippopotamus, however, is unique in spending most of the daytime in water or mud and feeding almost entirely at night or in the early morning. A study has been made of the structure of the epidermis of these animals and the differences found can, broadly speaking, be explained as modifications that are in accord with the animal's mode of life and surroundings. A more detailed study of the hippopotamus's skin shows that its structure, while probably primarily one of many possible modifications for an aquatic environment, permits a rate of transepidermal water loss that is greater than that in other animals for which figures can be obtained. No certain biological function can be proposed for this rapid loss of water; it is suggested that it is brought about by evaporation from the exposed layers of PAS‐positive material which in this skin are continuous from the Malpighian layer to the surface of an unusually thin stratum corneum.