Dislocation velocities have been measured on loops whose Burgers vectors have been determined by x‐ray topography. The new measurements do not agree with earlier work; the reasons for the discrepancy are discussed. The earlier work was also in error because of an incorrect assumption about the Burgers vector of the dislocations. The measured velocity‐stress behavior falls into two distinct stress regimes. At low stresses the dislocation velocity depends more strongly on stress than in the high‐stress region. The activation energy for dislocation motion depends on stress and varies from 2.9 to 1.5 eV over the stress range 0.6–8 kg/mm2.