Size measurements of dispersed rat bone crystals show that with age a greater proportion exceeds 400 A in length. The surface fractures of more heavily mineralized bones tend to be smooth and stepped, whereas those of less mineralized bones are fibrous. These observations combined with information reported elsewhere on the crystal-collagen relations in adjacent fibrils in turkey tendon, suggest that some crystals grow out of the confines of the collagen gap regions to form extended aggregates of large flat crystals with well developed fracture planes.