Changes in the ultrastructure of the toad epidermis during the moulting cycle are described on the basis of 17 skin preparations fixed in consecutive phases of the cycle. Our previous light microscopical findings that morphological changes are mainly restricted to a short period prior to and after shedding are confirmed. Differentiation of zonulae occludentes in the new replacement layer after shedding is described and discussed in relation to the changes in ion permeability after the moult. Changes in appearance and distribution of filaments and of two different types of granules during the moulting cycle are described and discussed in relation to current views on amphibian keratinization; it is concluded that the initial phase of keratinization in the toad is very rapid and with a high degree of synchrony, whereas the laying-down of interfibrillar, central dense matrix in the new stratum corneum takes up to 24 hours and is less synchronous. The separation of the old stratum corneum from the replacement layer is gradual; it may be accomplished by rupture of "pillars" bearing the desmosomal complexes between stratum corneum and the replacement layer, or by breaking within the desmosomes themselves. Observed changes in granular content of the replacement layer are considered of no importance for this process, since the time sequence of discharge into the subcorneal space is not correlated with the initiation of separation. Other possible mechanisms of separation are discussed.