Defines a new crystallinity index (CI), based on the x-ray powder diffractometric pattern of fossil bone carbonate hydroxylapatite. This CI value provides a semiquantitative way to estimate the diagenetic changes in archaeological and palaeontological bone phosphate. The studied samples are 71 human bones from the French Merovingian cemetery of Saint-Linaire (1,200-1,700 BP), 37 human bones from two necropolises in Niger (1,000-5,000 BP), two cave bear bones from Aldène Cave (southern France, around 70,000 BP), five bones of Miocene mammals, and 14 bones of Mesozoic reptiles. The main results follow: 1) no correlation exists between the increase in the CI and the age of the sample, even in one given locality; 2) the CI seems to be related to the taphonomical conditions in archaeological and fossil bones; 3) an increase in the CI is correlated with a loss in organic matter in one archaeological site, but the quality of preservation of organic matter in fossil bones does not depend on the changes in the CI; 4) the correlation between the increase in the CI and the decrease of CO₃ amount in bones from the Niger necropolises suggests a possible mechanism for the increase in crystallinity. The changes of crystallinity reflect early diagenetic changes and depend on the burial environments.