Abstract: Cesanite, Ca_1+xNa_4−x(SO₄)₃(OH)_x · (1 − x) H₂O, is hexagonal, space group P6₃/m, a = 9.446(1) and c = 6.895(1) Å, Z = 2. The crystal structure refinement, carried out by utilizing 457 independent reflections (final R = 0.021), shows that cesanite is isotypic to hydroxyapatite. The P atom of apatite is completely replaced in cesanite by an S atom; the two independent Ca sites of apatite are occupied respectively by Na and Na + Ca, the charge balance being by partial substitution of H₂O for OH⁻ although the occupancy of the (H₂O,OH⁻) sites is not complete. The relationship between the water vacancies and the structural features of the (Ca,Na) polyhedron is discussed. A new general formula for the apatite-wilkeite-ellestadite series is proposed.