SINGLE crystals of synthetic hydroxyapatite with minimum dimensions larger than about 0.1 mm have been required for X-ray and neutron diffraction investigations but they have not been available. The solid state exchange of chloride ions in single crystals of chlorapatite for hydroxyl ions, as reported here, produces suitably large single crystals of hydroxyapatite. The exchange holds additional interest both as a process and for the effects of a partial exchange on crystal properties.