An in-pile creep measuring apparatus was developed to obtain data on the compressive creep of ceramic fuel materials during the fission process. Experiments were conducted on annular specimens of unclad UO2 at fission rates up to 1.2 × 1013 fissions/cm3 sec, fuel temperatures in the range 1000 to 1300°C, and stress levels of 1000 to 4000 psi. The specimens were loaded in compression by a bellows-sealed gas-pressure-actuated piston. Specimen temperatures were controlled, both in-pile and out-of-pile, by electrical heaters. Axial deformation of the cylindrical specimens was continuously measured by a pneumatic displacement transducer.