As part of a large comparative study on the development of reptilian skin, we provide the first ultrastructural description of differentiation of the epidermis of the carapace and plastron in the Chelonia, using the Australian pleurodiran turtle Emydura macquarii as a model. The epidermis is initially composed of an external flat peridermis and a basal layer of cuboidal cells. During differentiation, the peridermis darkens, flakes off and is partially lost before hatching. Four to 6 layers of flat cells containing lipids and mucus form from the basal layer beneath the external peridermis. Because such cells are found only during embryogenesis, we have referred to these layers as embryonic epidermis. They contain reticulate bodies made of a meshwork of coarse filaments similar to those described in the inner peridermis of lizard and bird embryos. In advanced embryos, cells of the embryonic epidermis condense into a thin dark stratum which is subsequently lost after hatching. The lowermost 2 layers of the embryonic epidermis keratinise, as for a typical lepidosaurian α-layer. A splitting zone is progressively formed beneath the α-layer to separate the embryonic epidermis from the underlying β-layer. Patterns of cytodifferentiation of the β-synthesising cells over the carapace and plastron essentially resemble those of the lepidosaurian epidermis. The β-keratin matrix initially accumulates among ribosomes as round bodies not clearly surrounded by a membrane. These bodies appear not to be derived from the Golgi apparatus. Melanosomes and other dark granules of uncertain nature are present among early differentiating β-cells. The round β-keratin bodies merge with the dense bodies to produce the definitive variegated pattern of the mature β-keratin layer. The histochemistry suggests that calcium combines with organic molecules within β-keratinising cells to harden the tissue. In contrast to the β-keratin cells of lizards and snakes, cells of the mature β-keratin layer of E. macquarii maintain their cell boundaries in part or completely, a characteristics shared with the β-keratin layer of Sphenodon and crocodilians.