The organization of collagen fibrils in the human cornea and sclera was studied by scanning electron microscopy, after digestion of cellular elements by sodium hydroxide, and by conventional transmission electron microscopy. The collagen fibrils in the cornea had a uniform diameter of about 25 nm. In Bowman's layer, individual collagen fibrils were interwoven densely to form a felt-like sheet. In the stroma, most of the collagen fibrils ran abreast in lamellae, with varying widths and thickness. These lamellae were arranged basically parallel to the corneal surface but often communicated with those of adjacent layers by interchanging their fibrils. In the innermost stromal region adjacent to Descemet's membrane, collagen fibrils were oriented in various directions and interlaced, forming loose fibrillar networks. The sclera, however, was composed of collagen fibrils with various diameters ranging from 25-230 nm. Although these collagen fibrils formed bundles, they were not parallel but were entangled in individual bundles. These collagen bundles varied in width and thickness, often gave off branches, and intertwined with each other.