SUMMARYThe stacking fault energiesyof silicon (58 ± 6 mJ m−2) and germanium (75 ± 10 mJ m−2) were determined. Within the limits of accuracy γ was not found to change on doping with (13·8 mol m−3(8 × 1018cm−3) boron, and 1·17 mol m−3(7 × 1017cm−3) phosphorus). Freezing in dislocations under high shear stress reveals a different behaviour of screw dislocations: whereas these dislocations become wider in pure and p‐silicon, they become narrower in n‐silicon. From this we conclude the ratio of mobilities of the two 30° partials to be different in n‐ and p‐silicon. Other observations on frozen