The deformed microstructures of both irradiated and unirradiated Fe-9%Cr (Fe-9Cr) uniaxial tensile specimens have been examined to identify controlling mechanisms. Deformation following irradiation is found to occur in poorly defined channels about 100 nm wide, causing formation of discrete steps at surfaces and, interestingly, designated by nonuniformly distributed highly elongated voids. Deformation is by motion of a(0)/2 <1 1 1 > dislocations, which interact with and decompose irradiation-induced a(0) <1 0 0 > loops. The structure formed after extensive deformation consists of highly complex cell walls and moderate densities of individual slip dislocations. It appears that the deformation mechanism is channeling. (C) 2001 Elsevier Science B.V. All rights reserved.