Single crystals of a synthetic apatite-like phase with composition Na6.9Ca3.1(SO4)6(OH)1.1 weregrown under hydrothermal conditions. This compound crystallizes in the hexagonal space group P6–(a= 9.4434(13) Å, c= 6.8855(14) Å, Z= 1). The structure was solved by direct methods, andsubsequently refined using 655 independent reflections (R1 = 0.0542). The chemical compositionand the unit cell parameters indicated a close structural relationship with the mineral cesanite. Areinvestigation of the mineral showed that the natural and the synthetic phases are isostructural.Small differences result from the incorporation of both H2O and (OH)–into the structure of cesanite.Observed systematic absences revealed that the space group P63/mallocated to cesanite in earlierstudies is incorrect. The crystal structure of a cesanite with composition Na7.0Ca3.0(SO4)6(OH)1.0(H2O)0.8was successfully refined in space group P6– (a= 9.4630(8) Å, c= 6.9088(5) Å, Z= 1, R1 = 0.0468 for720 independent reflections [I> 2s(I)]). The symmetry reduction can be attributed to ordering of theNa and Ca atoms among four symmetrically independent cation sites.