Formation of the primary epidermis in freshwater triclad embryos was examined by transmission electron microscopy. After formation of the yolk-cell syncytium, the blasto-meres within it increased in number in the central region. At 57–40 h, most of them move peripherally as an elongate cluster. Oval cells occupying the frontal region of the cluster form precursor cells of the primordium of the primary epidermis. At 63–66 h, these precursor cells reach the surface of the yolk-cell syncytium and fuse to form a syncytium. The epidermal primordium then extends out over the surface of the yolk-cell syncytium. After 68–70 h, the primordium spreads to the opposite pole through the narrow space between the yolk-cell syncytium and its surrounding nonfused yolk cells. By 72–74 h, the spreading primary epidermis completely embraces the yolk-cell syncytium, suggesting that formation of the primary epidermis in the triclad embryo involves an epiboly-like movement of cells similar to that described for other neoophoran groups. We regard the primary epidermis as the organ of ectodermal nature. The formation and position of the blastopore in connection with germ-layer formation is discussed.