Dislocation structures characteristic of persistent slip bands were observed in the interior of polycrystalline copper after fatigue. At low strain amplitudes, within the plateau on the cyclic stress-strain curve, only structures identical to those seen in single crystals were observed. This allows the stress amplitude of the plateau to be calculated. At higher strain amplitudes, above the plateau, more complex structures were found. Further work will be needed to establish the nature of these new structures but it seems clear that the increase in stress amplitude is connected with their appearance.