The failure modes and dislocation substructure produced by fatigue crack propagation at 593°C in fast neutron irradiated and unirradiated Type 316 stainless steel were studied. The results indicate that the dislocation substructure produced by the crack propagation during both cyclic and combined cyclic-static loading conditions was directly dependent on the extent of matrix deformation as reflected by the mode of crack propagation. The results are consistent with the activation of either grain boundary or matrix mechanisms for deformation, as related to loading conditions, at the temperature investigated. The influence of neutron irradiation was to alter the operation of the basic deformation mechanisms by the increased difficulty of matrix dislocation motion resulting from the irradiation-induced defect production.