The detritus standing crop, microbial respiration, and macroinvertebrate biomass were examined in monthly samples from the riffle sections of a first—order woodland stream. Total detritus was remarkably constant; the average (with 95% CL) ash—free dry mass standing crop was 426.4 ± 85.9 g/m2over the 14 mo of the study. Throughout the year benthic detritus was dominated by fine particulate detritus (<1 mm), which made up 68.9% of the total ash—free dry mass. Woody debris made up 8%, whole leaves 3.5%, and leaf fragments and other coarse particulate detritus accounted for 19.7% of the total standing crop. Decreases in standing crop were attributable to microbial respiration, macroinvertebrate assimilation, and downstream export. Microbial respiration annually removed 150% of the average standing crop, with the major effect on the smallest particle size category. Macroinvertebrate assimilation, defined as the sum of respiration and growth, removed 11.6% of the detritus standing crop annually. Shredders accounted for 20% of total animal assimilation, with the remaining 80% attributable to collectors and grazers. Based on monthly changes, it appears that total detritus standing crop is the result of the past discharge regime, which determines the overall amount of detritus present, and the rate of biological (microbial and invertebrate) processes, which determine the size and quality of the detritus particles. This suggests that detritus in streams, while strongly affected by both biotic and abiotic factors, may be in equilibrium within physical and biological constraints such that an annual steady—state system exists, similar to that for soil systems.