Analysis of experimental data from coextruded Zr-2 clad uranium fuel rods irradiated either in NaK capsules or water at mean cladding temperatures below 325 to 350 C showed that during irradiation the cladding became plastically unstable resulting in localized longitudinal thinning, or necking and ultimate failure when the total circumferential strain caused by uranium swelling exceeded 1.5 per cent. At higher temperatures the circumferential strain before instability occurred was observed to increase rapidly until at 390 C strains up to 4 per cent were possible. Data from this irradiation test also showed that preirradiation striations in the cladding giving localized reductions of 10 and 30 per cent of the nominal thickness reduced the stable strain capability to 1.25 and 0.5 per cent, respectively.