The fatigue and creep-fatigue crack propagation performance of Type 316 stainless steel was investigated following fast neutron irradiation to a fluence level of about 5 x 10 to the 22nd power n/sq. cm (> O.1 MeV), about 26 displacements per atom (dpa), at 649 C. The results from fatigue tests of both annealed and 20 percent cold worked steel show that irradiation produced no significant effect on crack propagation rate when compared with unirradiated steel tested at 649 C. For tests conducted using creep-fatigue cycling, the inclusion of a two-minute hold time at the maximum cyclic load resulted in a marked increase in the crack propagation rate of the annealed steel. The effect of a one-minute hold time in the 20-percent cold worked, irradiated steel was to significantly increase the crack propagation rate at 649 C. Scanning electron microscope examination of the tested specimens revealed that the crack propagation mode of the specimens which exhibited increased crack propagation rates was primarily intergranular while a transgranular mode was observed for specimens with lower crack propagation rates. The results point toward a synergistic relationship between thermochemical history, precipitate formation, and hold time effects as the responsible mechanism for the crack propagation performance. (Author).