The compound cesium niobate, Cs 2 Nb 4 O 11 , is an antiferroelectric, as demonstrated by double hysteresis loops in the electric field versus polarization plot. The crystal structure refinement by X-ray diffraction at both 100 and 297K shows it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna, which is consistent with its antiferroelectric behavior. The 100-K structure data is reported herein. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Antiferroelectricity is produced by antiparallel displacements of niobium atoms along the c-axis at the phase transition temperature of 165°C. The critical field for onset of ferroelectric behavior in a single-crystal sample is 9.5kV/cm at room temperature.