The changes generated by a Spanish trout farm, located in the upper Rio Tajuna (Central Spain), on benthic macroinvertebrates were studied by comparing biological characteristics of an upstream station (S-1) with those of three downstream sites placed 0.01 (S-2), 0.15 (S-3) and 1 (S-4) km below the fish farm outlet . In addition, a biological index is presented for estimating relative contributions (informative weights) of major macroinvertebrate groups to the macrobenthic community . Species richness and Shannon diversity were depressed downstream from the trout farm . However, density and biomass values were significantly higher at downstream stations during the summer, presumably due to an increase in water temperature and food supply . Amphipods, plecopterans and planarians were the macroinvertebrates most adversely affected by the fish farm effluent. Coleopterans, ephemeropterans and trichopterans were absent immediately below the outlet (S-2), but exhibited a partial downstream spatial recovery of their informative weights at S-3 and S-4 . The abundance of tubificid worms, chironomids, simuliids and leeches increased below the trout farm, with dipterans predominating at all downstream sampling sites . The macrobenthic trophic structure was altered downstream from the trout farm by a significant increase in collectors (gatherers and filter feeders) and predators, and a marked decrease in shredders and scrapers . The highest environmental impact was found just below the troutfarm outlet (S-2) . It is concluded that the fish farm generates potamological effects on the functional structure of the macrobenthic community .