The structure and the mechanical properties of a newly developed highmodulus/high strength cellulose fibre spun from an anisotropic solution in phosphoric acid are discussed and compared with those of existing regenerated cellulose fibres. The crystal structure of the fibre is of the cellulose II modification, and the highly oriented and crystalline fibres have an initial filament modulus of 44 GPa, a sonic modulus of 58 GPa, and a strength of 1.7 GPa. It is shown that the mechanical properties of this fibre are well described by the continuous chain model. Lateral birefringence measurements and an electron diffraction study have established the orientation of the optical axes n(alpha) and n(beta) relative to the a and b axes in the crystal structure. Moreover, the most likely crack directions in the cellulose II structure have been identified.