The crystal structure of UGe2 has been determined by the X-ray precession method and Rietveld analysis of neutron powder diffraction data. The crystal system is orthorhombic (space group Cmmm) with lattice parameters a = 0.40089(1), b = 1.50889(3) and c = 0.40950(1) nm. The structure is isomorphic with one of the polymorphs of ThGe2.