The influence of high stress on the dissociation width of dislocations in doped Si and pure Ge was investigated. Whereas high B-doping (8.3 x 1018 cm-3) has no influence, in p-doped (7 x 1018 cm-3) crystals we find all screw dislocations narrowed ; i.e. both 30° partials have about the same mobility. 60° dislocations having their 90° partial in front are faster in this material and in Ge than are 60° dislocations with the reversed order of partials. In Ge a difference in mobilities of the partial dislocations is found too ; but the variation of the measured values is pronounced. The stacking fault energies of pure Si (γ = 58 ± 6 mJ/m2) and Ge (γ = 75 ± 10 mJ/m2) are redetermined