The crystal structure of Ba-5(OsO5)(3)Cl is described. Ba-5(OsO5)(3)Cl, M(r) = 1395.3, hexagonal, P6(3)cm, a = 10.928(2) Angstrom, c = 7.824(5) Angstrom, V = 809.2(6) Angstrom(3), D-x = 6.290 Mg/m(3). lambda(MoK alpha) = 0.71069 Angstrom, F(000) = 1170, room temperature, final R = 0.051 for 2900 observed reflections. The structure is isomorphous with Ba-5(ReO5)(3)Cl and is similar to that of standard chlorapatite; the PO4 tetrahedra are replaced by pyramidal OsO5 groups.