[en] The effect of fast-neutron irradiation on the fatigue-crack propagation and fracture toughness behavior of Alloy A-286 was characterized using fracture mechanics techniques. The fracture toughness was found to decrease continuously with increasing irradiation damage at both 24⁰C and 427⁰C. In the unirradiated and low fluence conditions, specimens displayed appreciable plasticity prior to fracture, and equivalent K/sub Ic/ values were determined from J/sub Ic/ fracture toughness results. At high irradiation exposure levels, specimens exhibited a brittle K/sub Ic/ fracture mode. The 427⁰C fracture toughness fell from 129 MPa√m in the unirradiated condition to 35 MPa√m at an exposure of 16.2 dpa (total fluence of 5.2 x 10²² n/cm²). Room temperature fracture toughness values were consistently 40 to 60% higher than the 427⁰C values. Electron fractography revealed that the reduction in fracture resistance was attributed to a fracture mechanism transition from ductile microvoid coalescence to channel fracture. Fatigue-crack propagation tests were conducted at 427⁰C on specimens irradiated at 2.4 dpa and 16.2 dpa. Crack growth rates at the lower exposure level were comparable to those in unirradiated material, while those at the higher exposure were slightly higher than in unirradiated material