The morphological sequence of events occurring in the development of the lateral-line organ was investigated in the embryo of the newt, Triturus pyrrhogaster, by means of electron microscopy. Six stages of development have been defined for convenience of description. The primordium of the lateral-line organ migrating from the pre- and post-auditory placodes is segmented into a small cell mass accompanied by a few Schwann cells enveloping nerve fibers. In the clump of cells of the lateral-line primordium, three kinds of cells are already distinguished by different degrees of darkness, i.e., dark, light and slightly dark cells (stage I). Both clump and Schwann cells enter the epidermis together (stage II). The two groups of cells are separated by the inner layer of the epidermis (stage III). The organ-forming cells elongate and the inner layer of the epidermis separates the Schwann cells from the clump (stage IV). The apexes of the organ-forming cells are exposed out of the epidermis (stage V). The lateral-line organ is almost matured. The dark cells correspond to type II of the supporting cells, the light cells correspond to type I of the supporting cells and the slightly dark cells correspond to the receptor cells because of the existence of cilia and a cuticular plate. The afferent nerve endings are found on the basal surface of the receptor cell (stage VI).