The lattice diffusion coefficient of 133Xe in cast uranium monocarbide was measured by postirradiation anneal experiments in the temperature range 1000 to 2000°C. The experimental results were analyzed by a small-time solution of Fick's law in which the effect of depletion of the surface layer due to recoil was incorporated in the initial distribution. The diffusion coefficient of specimens consisting of large grains (700 to 1000μ) was best approximated by in the range 1000 to 2000'C. Specimens with small grains (20 to 150μ) exhibited the same diffusion coefficient as the large grain samples above 1500°C. Below 1500°C, diffusivities in small-grained specimens varied widely, indicating dependence on grain size and, hence, the existence of appreciable grain-boundary diffusion.