The crystal structure of Valonia cellulose I beta is determined by X-ray fiber diffraction analysis. A careful reanalysis of two existing X-ray diffraction data sets for Valonia cellulose I leads to a consistent, definitive structure of the beta-phase. The results resolve ambiguities in existing X-ray analyses between "parallel up" and "parallel down" packing of the cellulose sheets in native Valonia cellulose. The molecular and sheet structures are essentially identical to those previously determined, but these results define precisely the packing of the sheets. The sheet packing is discussed in terms of the energy and density of the crystal structure. The structure of the beta-phase has specific implications for the packing of the alpha-phase of Valonia cellulose I.