Crystals of pentalanthanum pentatitanium heptadecaoxide (La(5)Ti(5)O(17) with 0.3% oxygen excess, or LaTiO(3.41)) have been synthesized by floating-zone melting, and the structure has been solved using single-crystal X-ray diffraction intensities. The monoclinic (P2(1)/c) structure consists of perovskite-like slabs of vertex-sharing TiO(6) octahedra, which are separated by additional oxygen layers. The slabs are five octahedra wide. Due to the adjustment of the TiO(6) octahedra to meet the coordination requirements of the La(3+) cations, a superstructure develops along the a axis.