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The paper investigates the dependence of the flow stress σ of ultra‐high‐purity molybdenum single crystals on temperature T, strain rate, and crystallographic orientation of the crystal axis. The cyclic‐deformation technique developed by Mughrabi and Ackermann allowed a complete set of flow‐stress data (covering the temperature range 125 to 460 K at 15 different shear strain rates, varying from 5.9 × 10—7 to 1.7 × 10—3 s—1) to be obtained on one and the same specimen. The two crystals investigated, with Schmid factors μ{110} = 0.50 and μ{112} = 0.43, or μ{110} = 0.40 and μ{110} = 0.43, respectively, had residual resistivity ratios exceeding 2.5 × 105, the main impurity being W. The data are in excellent agreement with the theory of flow‐stress control by kink‐pair formation and kink migration. It is shown that for both orientations the elementary glide steps of the a0 〈111〉/2 screw dislocations controlling the flow stress occur on {112} planes. The formation energy of a pair of isolated kinks on a {112} plane is 1.27 eV, in perfect agreement with the activation energy of the so‐called γ‐relaxation as determined by internal‐friction measurements. The apparent kink mass and the kink diffusivity could be determined, too. The present results leave no doubt that the “hump” observed in the σ–T relationship of high‐purity b.c.c. metals is due neither to a change in the glide mechanism nor to a special form of the Peierls potential but is a natural consequence of the dependence of the kink‐pair formation enthalpy on the resolved shear stress.