The molecular and crystal structure of the hydrated form of chitosan, which was obtained by deacetylation of chitin from crab tendon, was determined by the X-ray fiber diffraction method and the linked-atom least-squares method. The chitosan chains crystallize in an orthorhombic unit cell with dimensions a = 8.95(4), b = 16.97(6), c (fiber axis) = 10.34(4) Angstrom and a space group P2(1)2(1)2(1). The chain conformation is a 2-fold helix stabilized by O3---O5 hydrogen bond with the gt orientation of O6. The unit cell contains four chains and eight water molecules. There are direct hydrogen bonds (N2---O6) between adjacent chains along the b-axis, which makes a sheet structure parallel to the be-plane. These sheets stack along the a-axis. Each sheet is related to its neighboring sheet by 2(1)-symmetry along the b-axis. Water molecules form columns between these sheets and contribute to stabilize the structure by making water-bridges between polymer chains.