An electron-microscope study has been made of the nature of the dislocation arrangements produced during the second deformation of a copper single crystal previously deformed on another slip system. Two features are commonly observed. First, the second deformation commences by fine slip within the cell structure produced by the first deformation. This process, however, appears to be capable of accommodating only a limited strain, which depends markedly on the cell size, being much larger for larger cell sizes. At higher strains, coarse slip bands form, and it is suggested that these occur by the mutual annihilation of piled-up groups of positive and negative dislocations on opposite sides of cell walls, and thus creating softened regions which spread across the crystal.