Recent evidence indicates that a variety of antibiotic residues may affect the integrity of streams located downstream from wastewater treatment plants. Aquatic communities comprising bacterial and fungal decomposers and invertebrate detritivores (shredders) play an important role in the decomposition of allochthonous leaf litter, which acts as a primary energy source for small running waters. The aim of the present study was to assess whether an antibiotic mixture consisting of sulfamethoxazole, trimethoprim, erythromycin-H2O, roxithromycin, and clarithromycin has an effect on such a decomposer-detritivore system. Leaf discs were exposed to these antibiotics (total concentration of 2 or 200 microg/L) for approximately 20 d before offering these discs and corresponding control discs to an amphipod shredder, Gammarus fossarum, in a food choice experiment. Gammarus preferred the leaf discs conditioned in the presence of the antibiotic mixture at 200 microg/L over the control discs (pair-wise t test; p = 0.006). A similar tendency, while not significant, was observed for leaves conditioned with antibiotics at a concentration of 2 microg/L. The number of bacteria associated with leaves did not differ between treatments at either antibiotic concentration (t test; p = 0.57). In contrast, fungal biomass (measured as ergosterol) was significantly higher in the 200 microg/L treatment (t test; p = 0.038), suggesting that the preference of Gammarus may be related to a shift in fungal communities. Overall these results indicate that mixtures of antibiotics may disrupt important ecosystem processes such as organic matter flow in stream ecosystems, although effects are likely to be weak at antibiotic concentrations typical of streams receiving wastewater treatment plant effluents.