Copper single crystals subjected to a neutron dose of 3×1018nvt (total flux) at pile temperature have been examined after deformation by the following experimental techniques: (a) observation of the load‐extension relationships, (b) investigation of the slip‐line structure with the electron microscope, and (c) diffraction electron microscopy of thinned‐down single crystals before and after deformation. The critical resolved yield stress is in the order of 1.6 kg/mm2. In the early stages of deformation, the load‐extension curves show serrations which are as large as 1.0% of the critical resolved shear stress. In the linear portion of the stress‐strain curve, the rate of work hardening is less for irradiated single crystals than for the nonirradiated. The stress‐strain curves of the irradiated and the nonirradiated specimens are similar in the parabolic region of the curves. The slip‐line structure, at low deformations, consists of fine slip lines that are clustered together; the distances between the slip lines are, on the average, 100 A and often less; the distances between the clusters are in the order of 4&mgr;. This structure is quite different than the alpha‐brass structure, which in the past had been considered typical for irradiated copper. Cross slip, which is most abundant in the linear hardening region of the stress‐strain curve, is found to be orientation dependent. The slip‐line structures for the irradiated and nonirradiated crystals at high strains are very similar. Prismatic dislocation loops, apparently resulting from the condensation of vacancies, are found to be the most frequently produced radiation defect. The interaction between loops and glide dislocations results in heavily kinked dislocations which are probably responsible for the observed high yield stress. The glide dislocations were seen to remove the radiation damage. Because of this cleaning out of radiation‐produced defects and the ability of the dislocations to multiply from new sources, the prolonged ``easy glide range'' can be explained. Further, the proposed mechanism provides an explanation of the work hardening in the linear and parabolic parts of the stress‐strain curve.