The spherical shell method for investigating inelastic scattering cross sections has been used in a fast-reactor core environment. The changes in 238U/ 235U, 236U/ 235U, and 234U/ 235U fission ratios caused by placing shells of graphite, sodium, aluminum, iron, stainless steel, lead, and depleted uranium around the fission chambers were measured. Our studies show that reasonably accurate measurements can be made in a fast-reactor core. Where comparisons can be made, our results are in excellent agreement with the fission spectrum results of Bethe, Beyster, and Carter. Comparisons of our measured data with values calculated using two multigroup cross-section sets show clearly where these data sets are accurate and where they are in error.