Measurements of dislocation damping as a function of static biasing stress and during deformation have been carried out on single‐crystal, high‐purity tantalum. The results show that the dislocation mobility for small periodic stresses is orders of magnitude greater than for large constant stresses. The change in flow stress &Dgr;&sgr; due to the application of a small periodic stress &sgr;vis proportional to &sgr;v2. This second‐power dependence of &Dgr;&sgr; on &sgr;vis shown to result from thermal activation of the deformation. Calculations of activation energies and volumes using the point‐defect theories of Gilman and Fleischer give reasonable values.The large difference in dislocation mobility between the periodic type motion and the unidirectional motion indicates that the thermal component of the flow stress is not due to a Peierls‐type mechanism, but to discrete dispersed barriers to dislocation motion.