The deformation of single crystals of niobium purified by annealing in ultra-high vacua is studied. The stress-strain curves have 3 stages of hardening similar to those of f. c. c. crystals. The early stages of deformation are characterised by wavy slip lines on the top face and straight slip lines on the side face. Slip takes place on {011} or {121} planes, depending on the orientation of the crystal. The results of the orientation dependence of the operating slip planes show that the critical resolved shear stresses for slip on {011} planes and on {121} planes differ from each other, and that the latter depends on the sense of slip. The asymmetry of slip on {121} planes can be explained in terms of the 3-fold dissociation of screw dislocations. This mode of dissociation also explains the large lattice friction stress, its temperature dependence, and the elongation of glide loops along the Burgers vector direction