The bjarebyite group of minerals, characterized by the general formula BaX(2)Y(2)(PO(4))(3)(OH)(3), with X = Mg, Fe(2+) or Mn(2+), and Y = Al or Fe(3+), includes five members: bjarebyite BaMn(2+) (2)Al(2)(PO(4))(3)(OH)(3), johntomaite BaFe(2+) (2)Fe(3+) (2)(PO(4))(3)(OH)(3), kulanite BaFe(2+) (2)Al(2)(PO(4))(3)(OH)(3), penikisite BaMg(2)Al(2)(PO(4))(3)(OH)(3), and perloffite BaMn(2+) (2)Fe(3+) (2)(PO(4))(3)(OH)(3). Thus far, the crystal structures of all minerals in the group, but penikisite, have been determined. The present study reports the first structure determination of penikisite (barium dimagnesium dialuminium triphosphate trihydroxide) using single-crystal X-ray diffraction data of a crystal from the type locality, Mayo Mining District, Yukon Territory, Canada. Penikisite is isotypic with other members of the bjarebyite group with space group P2(1)/m, rather than triclinic (P1 or P-1), as previously suggested. Its structure consists of edge-shared [AlO(3)(OH)(3)] octa-hedral dimers linking via corners to form chains along [010]. These chains are decorated with PO(4) tetra-hedra (one of which has site symmetry m) and connected along [100] via edge-shared [MgO(5)(OH)] octa-hedral dimers and eleven-coordinated Ba(2+) ions (site symmetry m), forming a complex three-dimensional network. O-H⋯O hydrogen bonding provides additional linkage between chains. Microprobe analysis of the crystal used for data collection indicated that Mn substitutes for Mg at the 1.5% (apfu) level.