A model is developed to describe the tritium and fissile fuel flow in a fusion-fission system which consists of a fusion (hybrid) reactor, tritium production (fission) reactors, and (fission) power reactors. The fusion reactor provides all of the fissile fuel for the tritium production and power reactors. Tritium production reactors assure that the system is self sufficient in tritium even if the fusion reactor is not self sufficient. Studies were performed to determine the changes in the cost of electricity from the system as the tritium breeding responsibility varies between the fusion reactor and the tritium production reactors. Allowance for system growth was accomplished with the use of a compound doubling time parameter. Results indicate that the cost of electricity from certain fusion-fission systems may be comparable to that from other advanced systems expected in the same era.