A theory is derived for impurity‐controlled dislocation mobility and the resultant temperature variation of the flow stress of lithium fluoride. The theory is suitable to crystals in which the hardening is primarily due to tetragonal lattice distortions such as result from the introduction of magnesium into lithium fluoride, these being defects whose interactions with dislocations have been calculated previously. Dislocation motion is envisioned as a series of thermally activated dislocation jumps through the stress fields produced by individual solute atoms. The theory predicts the observed temperature variation of the flow stress within 25%, approximate values of the slopes of the velocity‐stress relations, and the activation energy for dislocation motion.