Data on the diffusion of fission krypton from irradiated 20 weight per cent uranium-aluminum alloy are presented. At temperatures below 640°C (the eutectic) there was no measurable loss of radiokrypton from this alloy during annealing periods of up to three weeks. At temperatures above the eutectic gas evolution occurred with a time dependence in rough agreement with the theoretical prediction for diffusion from spherical particles. The nature of the diffusion process for rare gases in metallic systems is discussed with particular reference to the limitations imposed on diffusion rate by solubility and available concentration gradient. The basic difference between the behavior of fission gases in dispersion-type nuclear fuels and in homogeneous solid-type fuels is outlined. The data on the uranium-aluminum alloy system are interpreted in light of this discussion.