Owing to some recent confusion in the literature the room temperature structure of LaAlO₃was redetermined by X-ray diffraction using a Guinier camera equipped with an image plate system. Different samples of pure LaAlO₃as well as a sample doped with 0.2 percent by weight ThO₂were used in the investigation. The space group of the structure, which is a rhombohedrally distorted perovskite structure, due to a rotation of the oxygen octahedra, is clearly R3c. The lattice constants of pure LaAlO₃are a = 5.3655(2) Å and c = 13.112(1) Å with a c/a-ratio of 2.444(1). The influence of doping on the lattice parameters and the tilting of the O₆-octahedron is discussed.