ELECTRON crystallography has two important advantages over X-ray crystallography for the determination of atomic positions in crystal structures : crystallographic phases can be determined directly from images(1), and extremely small samples can be analysed, On the other hand, the strong interaction between electrons and matter gives rise to dynamical effects(2) which complicate quantitative analysis of the experimental data, This has led to a pessimistic view of the possibility of direct crystal structure determination by electron crystallography(3,4), especially for compounds containing heavy elements. The atomic structures of a few inorganic crystals, mainly oxides, have been determined from electron microscopy images(5-7), but in no case was the atomic structure refined, Here we report the complete determination of an unknown structure, for the compound Ti11Se4, by electron crystallography. These results show that crystals that are too small for single-crystal X-ray diffraction and difficult to solve by powder diffraction may nevertheless be amenable to accurate structure determination by electron crystallography.