Abstract Nearly perfect zinc and cadmium single crystals were subjected to increasing tensile stresses and simultaneously characterized by transmission X-ray topography. Fresh dislocations were emitted in basal slip planes at resolved shear stresses as small as a few 104 Pa; in some crystals, an emission mechanism could be identified as the elongation and/or the expansion of original dipolar loops; in most eases however, the nucleation took place mainly from stress-enhanced parts of the specimens. Once emitted, the dislocations were observed to interact with sub-grain boundaries, prismatic loops of Burgers vector c or c + a, and with the crystal surface. A model is developed for the latter case, based on the dragging force felt by a dislocation at its point of emergence while it develops a slip step on the surface. In the rather thin crystals (80 pm) used for transmission X-ray topography, this surface effect, could be proved to be responsible for the locking of some fresh dislocations in their glide plane.