[en] New transmission electron microscopy evidence is presented for the dissociation of screw dislocations in Nb and for stacking faults enclosed in the dissociated dislocations. A weak-beam dark-field technique played an essential role in the attainment of the evidence. Despite the dissolution of moderate amounts of interstitial impurities in the foil matrix, screw dislocations are seen to display high mobility at temperatures above 20⁰C and a strong tendency to dissociate. The observed dislocation mobility, coupled with the reversibility of the slip traces, suggests that the present material involves little, if any, segregation of interstitial impurities in the stacking faults. (U.S.)