Abstract The crystallographic features of deformation twins in their early stages of development have been examined by transmission electron microscopy. The glide dislocations, predominantly of screw character, and an interspersion of faults and thin twins bounded by screw partials are observed within the region of localized slip. Only those dislocation segments which are in screw orientation appear to be involved in the formation of faults. Furthermore, the Burgers vector of the glide dislocations comprising the slipped region is always parallel to that of the partials bounding the faulted or twinned areas. This assesment is not in accord with the dissociation reaction proposed by Cottrell and Bilby (1951) for the nucleation of twins in b.c.c. crystals. These observations, however, appear to be consistent with Sleeswyk's hypothesis (1963) that three-layer twins could nucleate from the dissociation of screw dislocations. Conceivably a macroscopic twin could form when these three-layer twins grow into each other.