The temperature dependence of the critical resolved shear stress (CRSS), tau, of ultra-pure tantalum single crystals (RRR a parts per thousand yen 14000) observed below 250 K for a range of shear-strain rates was analyzed within the framework of a kink-pair nucleation model of flow stress. The CRSS/strain-rate data follow the model formulation, where C and D are positive constants, for each deformation temperature T in the range 78-250 K. Evaluation of the various slip-parameters of flow stress points to (211) slip system responsible for the yielding of ultra-pure tantalum single crystals in the so-called stress/temperature regime III (T < 250 K). The value of the pre-exponential factor in the Arrhenius-type equation for the shear-strain rate is found to be of the order of 10(5) s(-1), which is substantially lower than that determined in the stress/temperature regime II (250-400 K) and contradicts the assumption invariably made in most of the theoretical models of flow stress that is a constant over a wide temperature range.