Early phases of scale development in the chum salmon (Oncorhynchus keta) were studied by electron microscopy. The osseous matrix, composed of fine collagenous fibrils and dense ground substance, is secreted from cells of the scale papilla aggregated in the subepidermal connective tissue. The presence of dilated cisternae of the granular endoplasmic reticulum, rather numerous mitochondria, and abundant free ribosomes characterizes these osteogenic cells. Calcification of the matrix begins with the appearance of clusters of needle-shaped hydroxyapatite crystallites being limited within small dense particles scattered in the matrix. As the calcification progresses, they increase in number as well as in size. Thick collagen fibrils are added onto the lower surface of the calcified osseous layer by another kind of cells to form the fibrillary plate. These cells, rectangular in form, are characterized by a parallel array of highly extensive granular endoplasmic reticulum and well-developed Golgi zones with lysosomal bodies. The scale ridge arises as a partial thickening of the osseous layer. The ridge in cross section appears to be formed when an upper osteogenic cell is overlaid obliquely by the adjacent outer cell in the process of flattening of cells of the scale margin.