&tgr;0, the stress necessary to produce a plastic strain of 2×10−6, has been measured with the use of electrical resistance strain gauges in neutron‐irradiated copper single crystals as a function of neutron dose (∼1016to 1020neutrons/cm2) and testing temperature (77° and 300°K). &tgr;0increases from about 0.3 kg/mm2for ∼1016neutrons/cm2to over 5 kg/mm2for ∼1020neutrons/cm2; in annealed copper crystals, &tgr;0is ∼0.02 kg/mm2(previously determined by other researchers). At all neutron doses, &tgr;0is about 75% of the macroscopic flow stress. &tgr;0is observed to increase with the cube root of the neutron dose; the macroscopic flow stress shows the same dose dependence.The results are explained on the basis of dislocation generation and obstacle cutting mechanisms controlling the yielding. These mechanisms are thought to be simultaneously operative.