The suitability of flux- and bilinearly weighted collapsing schemes for fast-reactor transient analyses was studied. Comparisons of few-group transient results were made with “exact” 26-group and three-trial-function energy synthesis results. Normal flux weighting was not satisfactory even when up to 12 groups were used. Bilinear schemes with only 6 or 8 groups, using unperturbed fluxes and perturbed adjoints (or vice versa), were found to be satisfactory. When reactivities were less than one dollar, unperturbed fluxes and adjoints were sufficient to give satisfactory results. With bilinear group collapsing, it is necessary to consider discontinuities arising at interfaces between regions where the flux and adjoint spectra are different and to use modified continuity conditions.