FUELCYC is a code, written in the Fortran II language for a 32,000-word IBM 704 computer, which has been developed for the study of fuel cycles in large cylindrical thermal power reactors. It is a two-dimensional diffusion-theory code in which it is assumed that the reactor core is symmetrical in the azimuthal direction. Core properties such as the fuel composition, burnup, flux and power-density distributions, and reactivity can be calculated throughout the life history of the fuel for different methods of managing fuel and control poisons. Only a few minutes of computer time are required for a complete fuel-cycle analysis of the life history of the core.