An investigation on the crystallisation of a whitlockite type orthophosphate phase in a calcium phosphosilicate glass frit with the molar composition (3CaO.P2O5)(20)(SiO2)(35)(MgO)(38.354)(K2O)(6.646) was made. Isothermal heat treatments at different temperatures combined with XRD and SEM analysis of the glass ceramics have shown that in glass frits of this composition the whitlockite type orthophosphate phase, Ca9MgK(PO4)(7) develops from earlier crystalline phosphate phase precursors, namely from oxyapatite (OAp), the ideal composition being Ca-5(PO4)(3)O. This OAp phase seems to evolve from a previous tricalcium. phosphate phase, Ca-3(PO4)(2) (TCP).