Mixed (U,Pu)C fuel pins of both pellet and sphere-pac designs have been irradiated in epithermal and fast fluxes. The results of postirradiation examination allow a limited comparison of the two designs to be made. Two experiments were performed, one in the Belgian Reactor 2 at Mol, Belgium and the other in the United Kingdom ’s Dounreay Fast Reactor. In each experiment the two fuel designs were coirradiated. Peak burnup of 7.7% was achieved at ratings up to 95 kW· m−1. In each experiment one of the pellet pins failed. Postirradiation examination revealed minor differences in the fuel pin behavior in regard to gas release, clad carburization, and plutonium movement. However, fuel restructuring and the pattern of pin strain were markedly different in the two designs and resulted in the failure of two of the pellet pins. The poorer performance of the pellet pins was due to the design—high bulk density fuel and large fuel clad gap.