To minimize waste disposal problems associated with the residual radioactivity of the first wall material of a fusion reactor, fast induced radioactive decay (FIRD) alloys based on the Fe-Cr-Mn system are being investigated. The objective of this research was to evaluate the effects of irradiation on cyclic strain localization and fatigue crack initiation in a FIRD Fe-12Cr-20Mn alloy and to compare the response to commercially available 316 stainless steel. The alloys were irradiated with 200 keV Fe ions to a dose of I x 1016 ions/cm 2 and 15.5 keV He ions to a dose of 7 • 1015 ions/era 2 to simulate the irradiation-induced defect structure and helium concentration that would be produced in a fusion reactor. Irradiated specimens were fatigued in a cantilever beam fatigue testing machine with the deflection set to produce a fully reversed total strain amplitude of 0.25 % on the surface of the specimen. Acetate replicas were obtained during the fatigue tests to provide a record of surface fatigue damage. Transmission electron microscopy (TEM) analyses were performed to characterize the microstructural changes resulting from the irradiations and interactions between fatigue-induced glide dislocations and the irradiation-induced defects.Results indicate that the irradiated Fe-Cr-Mn alloy exhibits fatigue properties similar to 316 stainless steel. Glide dislocations produced by fatigue cycling annihilate irradiation-induced defects. The defect annihilation causes the formation of cleared channels in which the cyclic plastic strain is localized. Subsurface slip bands penetrate the irradiated regions through the cleared channels and serve as fatigue crack initiation sites.