AbstractContinuous slowing down theory is applied to the treatment of the elastic moderation of neutrons in a fast-reactor assembly, where strong scattering resonances are prevalent. Three prescriptions for the moderating parameters, one of which is original to this paper, are studied. Continuous slowing down theory is demonstrated to yield quite accurate results for the neutron spectrum, when the improved prescriptions presented in this paper are used. A method for treating narrow resonance absorption separate from the slowing down calculation, then using the results of the former to attenuate the flux obtained from the latter, is presented and demonstrated to be promising.