Fatigue specimens were prepared from Nimonic PE16 fuel assembly components irradiated in a Commercial Advanced Gas-Cooled Reactor (CAGR) at temperatures in the range 631 K to 916 K and integrated fast neutron doses up to 21 x 1024 neutrons/m2. The tests were performed in air at temperatures from 623 K to 973 K at a frequency of 10 Hz and R-ratio (minimum/maximum load) of 0.2. Crack growth rates were correlated with the stress intensity factor range using the relation da/dN — CΔKm. The threshold stress intensity factor range, ΔKth) obtained in load-shedding tests was in the range 6 to 9 MN/m3/2 and was not markedly influenced by neutron dose, irradiation temperature or test temperature. Specimens having both uniform fine grained and non-uniform coarser grained microstructures gave similar Δth, so that roughness-induced crack closure was not a major factor, despite wide differences in crack path morphology. In tests at 973 K, somewhat higher values of ΔKth were obtained which were probably a consequence of oxide-induced closure.