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 — CΔK^m. The threshold stress intensity factor range, ΔK^th) obtained in load-shedding tests was in the range 6 to 9 MN/m^3/2 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 Δ^th, 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 ΔK^th were obtained which were probably a consequence of oxide-induced closure.