Winter stoneflies (Zapada oregonensis, Z. cinctipes and Capnia confusa) used detritus as a nymphal food almost exclusively and dominated the shredder biomass in a third-order mountain stream in Colorado. Laboratory feeding trials showed that ingestion rates and fecal production were temperature-dependent, although not greatly different between aspen or alder leaf discs. Assimilation efficiency (AE) was not temperature-dependent. Higher AE values for alder indicate that it is superior to aspen as a food source. High consumption rates (ca. 30% of body weight/day) by the shredders, even at low temperatures, would result in the conversion of considerable quantities of leaf litter to finer particles. However, size fraction analysis of egested material indicated that particle size reduction by shredder processing only partially explains detrital composition in a stream dominated by fine-particle feeding detritivores.