Variations in the fine structure of the flask cells of frog epidermis are described. Some flask cells have apical ridges with an alveolar layer of material under the cell membrane; others may have adjacent apical ridges sealed together by fusion of the outer leaflets of the cell membrane. The apical membrane of the flask cells and of the second tier epithelial cells (replacement or first reactive cells) is visibly different from the membranes on the lateral and basal aspects. Other variations in the appearance of flask cells may depend on their individual age, or on the stage of the moult cycle. The events of moulting can be divided into preparation, separation, sloughing and differentiation phases. During the preparation phase the flask cell apices disintegrate, and they are covered over by the adjacent replacement cells, which will later keratinize during the separation phase. New tight junctions are formed in the outer layers of the epidermis. Desmosome release frees the slough during the separation phase, before it is actually shed. During the differentiation phase the flask cells develop new apical ridges. The fine structure of the flask cells is compared with that of the mitochondria rich cells of the bladder epithelium, and with mitochondria rich cells which have been found in the palatal epithelium; the flask cells are considered not to be homologous with these other types. There are no comparable cells in the corneal epithelium. Possible functions of the flask cells are considered; some previous hypotheses can now be discounted. An attempt is made to integrate the phases of the normal moult with the variations in active sodium transport and water flux reported from isolated skin treated with aldosterone. It is suggested that the flask cells may transmit a hormonal stimulus causing release of the slough, but that they are not concerned in the effects on sodium transport, which can be explained by changes of the epithelial cells. The flask cells could however be involved in the transport of substances other than sodium.