The crystallinity of bone mineral at different stages of maturation has been measured by quantitative X-ray diffraction methods. Crystallinity measurements were made on tibial mid-diaphyses from 17-day embryonic chicks, newly-formed periosteal bone from embryonic chicks, and density-fractionated bone from post-hatch chickens from 5 weeks to 2 years of age. For a given animal age and degree of mineralization, crystallinity increases with animal age, indicating that changes in bone mineral occur even after mineralization is complete or nearly complete.