A component of the basal plate which has a plywood-like organization similar to that of the elasmoid scales of teleosts is described in the scales of Polypterus senegalus for the first time. The origin and development of this structure is studied in young (50-117 mm, standard length) and adult (225 and 240 mm) specimens using light and electron microscopy. In 50 mm fish, the scales are imbricated and composed mainly of a succession of orthogonal collagen layers forming a plywood-like structure, the isopedin. The outer surface of the scale is ornamented locally by irregular patches of collagenous material. The layers are not mineralized, whereas the superficial patches are well calcified. The isopedin thickens until it has 12-15 layers and then stops growing (88 mm fish). It mineralizes irregularly from its upper part, and two vascular regions, surrounded by woven-fibered osseous material, form on the outer and deeper surfaces of the isopedin. These regions thicken while the vascular canals close by centripetal deposition of parallel-fibered osseous tissue. The outer region is the superficial part of the mature scale (called here osteodentin), which is covered by the ganoine deposited by the epidermal cells. The deeper part constitutes the definitive basal plate, composed of parallel-fibered osseous tissue. The results show 1) that the young ganoid scales of Polypterus senegalus have a structure similar to that of typical elasmoid scales; and 2) that the isopedin structure does not change during ontogeny and so represents a permanent record of the first ontogenetic stages. The phylogenetic implication of these results is that the elasmoid scales of teleosts arose by a process of paedomorphosis.