Spiders were the most important entomophagous predators of a 137Cs—tagged Liriodendron tulipifera decomposer community. Radioisotope techniques were used to quantify the bioenergetic impact of spider predation on the forest—floor detritus food web. Temperature dependent 137Cs elimination by spiders and 137Cs concentrations in field animals were used in intake—loss equations to estimate ingestion by the spider population. Radiocesium body burdens fluctuated seasonally in response to litterfall and seasonal differences in temperature and precipitation. Ingestion rates, assimilation efficiencies, and respiratory energy losses were used to estimate gross and net production of the forest spider population. Annual mean population density of spiders was 126 individuals/m2. Population—density peaks occurred in fall, early winter, and summer. Mean biomass for the year was 43 mg dry wt/m2; maxima occurred in early winter and spring. Spiders 1—10 mg in size accounted for 59% of total biomass; spiders <1 mg or > 10 mg comprised the remaining 24% and 17% of the population, respectively. Predicted values for ingestion, assimilation, and respiration indicate that forest—floor spiders were active throughout a temperature range of 5°—20°C, with 13.3° the optimum for net production by spiders of all size classes. Spiders killed an equivalent of 2.08 kcal prey tissue/m2 per year and ingested 1.68 kcal/m2 per year. Respiratory energy loss equaled 1.23 kcal/m2 per year. Trophic level efficiencies calculated from seasonal energy—budget data were: (1) intake efficiency (prey killed/ingestion), 73—86%; (2) assimilation efficiency (assimilation/ingestion), 90—99%; (3) respiration efficiency (respiration/ingestion), 58—96%; (4) respiration/assimilation, 63—106%; (5) ecological growth efficiency (net production/ingestion), —5—34%; and (6) tissue—growth efficiency (net production/assimilation), —4—37%.