The crystal structure of beta-D-cellotetraose shows the same molecule packing as cellulose II, with two antiparallel molecules in the unit cell: For cellulose II, the orientation of the C6-O6 bonds has been described as gauche-trans and trans-gauche, respectively, for the two antiparallel molecules, which otherwise have identical conformations. In contrast, in beta-D-cellotetraose all C6-O6 bonds are gauche-trans, but the conformations of the two antiparallel molecules are different. Energy minimization and molecular dynamics studies suggest that the structure of cellulose II should be reinvestigated in light of these findings.