The organization of collagen fibrils differs in woven bone and lamellar bone, and it reflects certain aspects of the nature of the mineral crystals associated with them. In order to investigate the morphology and distribution of apatite crystals in woven bone, mineralized collagen fibrils and isolated crystals from the mid-diaphyses of human fetal femurs were observed with scanning and transmission electron microscopy and high-resolution electron microscopy. A number of features of woven bone were observed for the first time by these means. Similar to mature crystals from lamellar bone, the apatite crystals in woven bone are also platelet-shaped. However, most likely because of a high rate of old bone resorption and new bone formation in woven material, the average crystal dimensions are considerably smaller than those of mature crystals in lamellar bone. Apatite crystals were noted on the surface of collagen fibrils in woven bone. In densely packed woven bone, the periodicity of mineral deposited on individual fibrils is in registration over many fibrils. In addition to their association with collagen surfaces, crystals also appear distributed in both extrafibrillar and intrafibrillar collagen regions. In both cases, the minerals are crystalline and defect-free. These characteristics provide insight into the spatial and temporal relation between collagen and mineral that is the basis for the structure and organization of the mineral comprising human woven bone.