Crystals of barium monogallate were obtained by growth from a stoichiometric melt at 13503C. The hexagonal crystals belong to space group P6 3 , a β«Ψβ¬ 18.6403(7) A s and c β«Ψβ¬ 8.6801(2) A s , V β«Ψβ¬ 2611.9(2) A s 3 , Z β«Ψβ¬ 24, D calc. β«Ψβ¬ 5.20 g cm Ψ3 . The structure was solved by Patterson techniques using a single crystal di4raction data set. Merohedral twinning of the crystal (twin element 2 [110] ) was accounted for in the re5nement calculations (wR2 β«Ψβ¬ 0.056 for 4183 re6ections). The main structural features of BaGa 2 O 4 are layers of six-membered rings of GaO 4 tetrahedra perpendicular to the c-axis. The stacking of the layers parallel to c results in a three-dimensional framework containing tunnels, where the barium cations are located. Within a single layer two di4erent types of ditrigonal-shaped six-membered rings are distinguished on the basis of relative orientation of up (U) and down (D) pointing apices of adjacent tetrahedra: one-fourth of the rings have an UDUDUD topology, whereas the sequences of directedness of the remaining rings is UUUDDD. The structure is isotypic with KAlGeO 4 . It is the 5rst member of this structure family containing exclusively trivalent tetrahedral ions of only one chemical species and divalent nonframework cations within the channels.