The solid mineral phases of calcium-phosphate (Ca-P) in the long bones from embryonic chicks between the ages of 9 and 13 days have been examined by high voltage (1.0 MV) electron microscopy and electron microdiffraction. The study was undertaken to identify the chemical and crystallographic nature of the inorganic mineral phase(s) prepared under conditions which significantly reduce artifacts of specimen preparation and microscopic examination of the tissues. Electron microdiffraction patterns of solid mineral phase particles in the osteoid matrices of the subperiosteal region of tibiae were principally those of poorly crystalline hydroxyapatite. In rare instances (less than 1% of the estimated volume of the mineral phase present in the zone of early mineralization), relatively large single crystals were observed within clusters of hydroxyapatite. From calculations of both interplanar spacings and measurements of angular displacement of diffraction reflections from single crystal microdiffraction patterns, two distinct phases other than hydroxyapatite were identified: brushite and beta-tricalcium phosphate. A third phase, resembling an apatite, remains unidentified. The results suggest that very small amounts of nonapatitic phases of Ca-P exist in chicken bone tissue. No temporal relationship could be established between the nonapatitic and apatitic phases. There is at present no evidence from this study to support the concept that nonapatitic phases are precursors of a final apatitic phase in bone.