The interfollicular epidermis (IFE), excluding the hair follicles, of the back of the adult normal mouse was examined with the electron microscope. The phase of the hair follicular cycle and the mitotic state in the skin were taken into account. Careful attention was paid to the experimental technique and possible sources of error. The IFE of the mouse is, in principle, composed of one layer of nucleated cells of the basal-cell type. These parental cells neither move upward nor keratinize. Instead the majority of the daughter cells immediately lose their nuclei, while the keratinization proceeds. Occasionally, disintegration of the nucleus of the daughter cell is delayed so that two nucleated cells one upon the other are observed. If, however, mouse IFE contained two or more cell layers over a more extensive area, this was obviously due to accidental factors such as inflammation or fighting. The basal cells of IFE, over a wide basal surface, are fixed to the dermoepidermal junction by special attachment devices, the ultrastructure of which is also described. The boundaries of adjacent cells intimately follow each other in an intricate, folded, curtainlike fashion. Cytoplasmic pseudopodia worm into corresponding passages of the neighboring cells. Neither intercellular spaces nor intercellular bridges occur between basal cells in mouse IFE, but do occur between the overlying differentiating cells. The ultrastructure of mouse IFE is independent of the phase of the hair follicular cycle. The mitotic state was the same in all material examined. The mouse IFE differs from the multilayered one in the human, rat, or guinea pig, as described in the literature.