The deformed microstructures of both irradiated and unirradiated Fe-9Cr uniaxial tensile specimens have been examined to identify controlling mechanisms. Deformation following irradiation is found to occur in poorly defined channels, causing formation of discrete steps at surfaces and delineated by nonuniformly distributed highly elongated voids. Deformation is by motion of a∕2<111> dislocations, which interact with and decompose irradiation-induced a<100> loops. The structure formed after extensive deformation consists of highly complex cell walls and moderate densities of individual slip dislocations.