Biodynamic modeling was used to investigate the uptake and bioaccumulation of arsenic and silver from water and food by two Hydropsychid caddisfly larvae: Hydropsyche siltalai and Hydropsyche pellucidula. Radiotracer techniques determined the uptake rate constants of arsenic and silver from water, and assimilation efficiencies from food, and their subsequent loss rate constants after accumulation from either route. The uptake rate constants (±SE) of As and Ag from solution were 0.021±0.005 and 0.350±0.049Lg(-1)day(-1), respectively, for H. siltalai, and 0.435±0.054 and 0.277±0.021Lg(-1)day(-1), respectively, for H. pellucidula in moderately hard synthetic water at 10°C. The assimilation efficiencies (±SE) of As and Ag from radiolabeled ingested food were 46.0±7.7% and 75.7±3.6%, respectively, for H. siltalai, and 61.0±4.2% and 52.6±8.6%, respectively, for H. pellucidula. Ag, but not As, AEs were significantly different between species. The AE of Ag differed from the AE of As in H. siltalai, but not in H. pellucidula. Mean efflux rate constants after accumulation of metals from solution or food ranged from 0.039 to 0.190day(-1). The efflux rate constants of As and Ag accumulated from solution were significantly lower than those of As and Ag assimilated from ingested food in both species. Experimentally derived ku and ke values were then used to predict As and Ag tissue concentrations in hydropsychids collected from 13 UK sites, including metal-contaminated streams in Cornwall. Arsenic and silver concentrations in environmental water and food (suspended particles) samples were measured. Biodynamic models successfully predicted accumulated As and Ag concentrations in resident H. siltalai and H. pellucidula at each site. The models also showed that more than 95% of accumulated As and almost 100% of accumulated Ag in H. siltalai and H. pellucidula are derived from ingested food rather than from water.