The crystal structure of hexagonal barium titanate has been determined by X-ray methods. The unit cell, which contains six units of BaTiO3, has a0 = 5.735 A. and c0 = 14.05 A. In the space group C63/mmc all the ions occur at special positions: 2 Ba at (b), 4 Ba at (f), 2 Ti at (a), 4 Ti at (f), 6 O at (h), and 12 O at (k). The structure is built up of six close-packed layers of Ba and O ions, each layer consisting of one Ba to three O ions, having the packing sequence ABCACB. The Ti ions are located in the oxygen octahedral holes between the layers. The structure is remarkable in that two-thirds of the TiO6 octahedra occur in pairs which share a face to form Ti2O9 co-ordination groups. The compensating distortion which occurs in the Ti2O9 groups increases the Ti—Ti distance by 0.33 A. while the O—O distance in the shared face is decreased by 0.38 A. The existence of Ti2O9 groups in hexagonal barium titanate, an unusually stable substance under these circumstances, constitutes a notable exception to certain of Pauling's rules for complex ionic structures.