Native cellulose has been shown to consist of a crystalline array of parallel chains, based on the X-ray diffraction data for specimens from the sea alga Valonia ventricosa. The unit cell is monoclinic with dimensions a = 16.34 Å, b = 15.72 Å, c = 10.38 Å (fiber axis), and β = 97.0°. The space group is P21 and the cell contains disaccharide segments of eight chains. Models containing chains with the same sense (parallel) or alternating sense (antiparallel) were refined against the intensity data using rigidbody least squares procedures. The results show a preference for a parallel chain structure with specific chain polarity with respect to the c axis. The refinement places the CH2OH side chains approximately 20′ from the so-called tg conformation, with a result that an 02′H…06 intramolecular bond is formed. The structure also contains an 03H…05′ intramolecular bond and an 06H…03 intermolecular bond along the a axis. All these bonds lie in the 020 planes, and the structure is an array of hydrogen-bonded sheets. A major consequence of this work is that regular chain folding can be ruled out and cellulose is seen as extended chain polymer single crystals.