The average velocities of dislocations have been measured in four semiconductor crystals‐silicon, germanium, gallium antimonide, and indium antimonide. The range of measured velocities was between 10−6and 2×10−1cm/sec. The dislocation velocities in these crystals are relatively insensitive to stress when compared to LiF or Si‐Fe. The effect of temperature, in the range 0.5 to 0.8TM, on the velocities at various constant stresses was measured and the activation energies for the dislocation velocities were obtained in each of the crystals. No marked dependence of activation energy as a function of the applied stress was obtained.The effect of strain on the dislocation densities was measured as a function of orientation, temperature, and strain rate. It was found that even in initially dislocation‐free crystals, dislocations were generated at quite low stresses. From direct measurements of etch pits and dislocation velocities it was possible to estimate the fraction of mobile dislocations. Under certain conditions the percentage of mobile dislocations was as low as 3 to 8%.