Precipitates of calcium phosphate were obtained from aqueous media, some of which contained Mg2+ and F−. Initial solids removed as a slurry were shown by electron diffraction to be amorphous when Mg2+ and F− were not present. Subsequent ageing for 24 h in the supernatant induced a change in morphology and diffraction pattern as observed with the electron microscope. The final solid was crystalline hydroxyapatite. In the presence of F− the initial precipitate was crystalline phase. Conversely, the presence of Mg2+ in the system markedly delayed the transformation from the amorphous to the crystalline phase. When both Mg and F were present the initial phase was amorphous but transformation occurred much more rapidly than in systems with Mg alone. Examination of the solids at high magnification in the electron microscope suggested that the spheroidal structures of the amorphous phase may play a role in the transformation by aligning in a chain-like manner, which appeared to be a morphological feature of the crystalline phase.