The structure of the aluminum borate CaAl2(BO3)(2)O has been established by single-crystal X-ray diffraction methods. It crystallizes in the trigonal space group c(h) (Z = 6) with unit-cell parameters a = 4.810(6) Angstrom, c = 46.633(5) Angstrom, and V = 934(1) Angstrom(3). The structure was determined from 274 unique reflections and refined to the final residuals R = 0.024 and wR = 0.032. It is characterized by an association of BO3 triangles, CaO6 octahedra, AlO4 tetrahedra, and unique, two-coordinate O atoms. Structural comparisons are made to Na+ beta-alumina.