The ultrastructure of the skin of air-adapted mammals (bats) is not known. The study at the electron microscope of the skin of the back and the flying membrane of Pipistrellus kuhlii showed that the thickness of the epidermis is very low (10-12 μm), and that 1-2 flat spinosus cells are present beneath the stratum corneum which is formed by very thin corneocytes that resemble those of avian apteric epidermis. The stratum granulosum is discontinuous and few small (less than 0.3 μm large) keratohyalin granules are present. The epidermis is reduced to one flat basal layer in contact with the stratum corneum in many areas of the flying membrane. Transitional corneocytes are almost absent suggesting that the process of cornification is very rapid. In the basement membrane numerous hemidesmosomes are present and form attachment points for the dense dermis underneath. Numerous collagen fibrils directly contact with the hemidesmosomes and the dense lamella of the basement membrane. Sparse elastic fibrils allow the stretching of the epidermis during flight and the rapid folding of the epidermis after flying without damaging the epidermis. Like in avian epidermis, the production of lipids is high in bat keratinocytes, and multilamellar bodies discharge lipids extra- and intra-cellularly. This may compensate the lack of a thick fat layer in the dermis of the flying membrane as lipids may help in thermical insulation against the cooling air currents flowing on the bat skin during flight. Fur hairs are very thin (4-7 μm), and they have an elaborated cuticle made of pointed expansions similar in texture with that of the cortex. Cuticle cells form hook-like grasping points that allow to keep hairs stuck together. In this way the pelage remains compact in order to maintain body temperature.