The crystal and molecular structure together with the hydrogen-bonding system in cellulose Ibeta has been determined using synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from tunicin. These samples diffracted both synchrotron X-rays and neutrons to better than 1A resolution (>300 unique reflections; P2(1)). The X-ray data were used to determine the C and O atom positions. The resulting structure consisted of two parallel chains having slightly different conformations and organized in sheets packed in a "parallel-up" fashion, with all hydroxymethyl groups adopting the tg conformation. The positions of hydrogen atoms involved in hydrogen-bonding were determined from a Fourier-difference analysis using neutron diffraction data collected from hydrogenated and deuterated samples. The hydrogen atoms involved in the intramolecular O3...O5 hydrogen bonds have well-defined positions, whereas those corresponding to O2 and O6 covered a wider volume, indicative of multiple geometry with partial occupation. The observation of this disorder substantiates a recent infrared analysis and indicates that, despite their high crystallinity, crystals of cellulose Ibeta have an inherent disorganization of the intermolecular H-bond network that maintains the cellulose chains in sheets.