A description is given of two methods for collapsing many-group cross sections to few-group cross sections. The analysis is applicable to intermediate and large-volume fast reactors. It is shown that the simple flux weighting collapsing recipe—with reciprocal weighting of the transport cross section—is adequate for computing reaction rates, keff, etc., but inadequate for computing moderator reactivity coefficients. A new collapsing algorithm is developed which permits an accurate calculation of moderator reactivity coefficients using few-group cross sections.