The gill filament growth and the secondary lamellae formation are examined in Teleostei fishes, particularly in Salmo irideus Gibb, embryos. The gill filament apex is ogee-shaped, and the new secondary lamellae are in various developmental stages, arranged alternately on the two sides of the gill filament, the more recently formed ones being in a distal position. The afferent and efferent vessels, which lay along the filament edges, join at the apex. The wall of these vessels is bordered by typical endothelial cells in the area underlying the epithelium and by a mass of line-ranged cells in the axial position of the filament. These cells are immature pillar cells, i.e. pilastroblasts (Acrivo), which appear serrated with one another. In the area below the apex, where the respiratory lamellae begin to differentiate and to project towards the gill filament surface, the afferent and efferent vessels appear to be surrounded by typical endothelial cells instead of separated by pilastroblasts. Under the electron microscope the developing pillar cells reveal a fine structure characterized by large nucleus, scanty cytoplasm, very numerous free ribosomes. Our observations suggest that the pilastroblasts must be considered a sort of endothelial cell «reservoir», which are not involved in collagen production.