Benthic macroinvertebrate response to organic pollution generated by a trout farm, located in the upper Rio Tajuña (Central Spain), was evaluated by calculating Whittaker's and Camargo's dominance indices; Margalef s, Menhinick's, Camargo's, Shannon's and MacArthur's diversity indices; and several biotic indices, including the Trent Biotic Index (TBI), the Chandler Biotic Score (CBS), the Biological Monitoring Working Party (BMWP), the Average Score Per Taxon (ASPT) and the Overall Quality Rating (OQR). An upstream sampling site (S-1) and three downstream stations placed 10 (S-2), 150 (S-3) and 1000 (S-4) m below the fish farm outlet were selected along the study area. Site S-3 was situated just below a man-made waterfall (1.8 m in height). Winter, spring and summer macrobenthic surveys were conducted during 1986. Plecopterans, ephemeropterans, coleopterans, trichopterans, amphipods and planarians decreased in abundance or were basent at downstream sampling sites, whereas simuliids, leeches, chironomids and tubificid worms increased in abundance. All diversity and biotic indices had highest and lowest values at S-1 and S-2, respectively, and higher values at S-3 (below the waterfall) than at S-4. However, Shannon's and MacArthur's diversity indices and the TBI were less effective in quantifying differences among sampling sites, exhibiting the lowest positive coefficients of linear correlation with species richness and other diversity and biotic indices. Both dominance indices had highest values at S-2, but similar values at S-1, S-3 and S-4. Biotic indices (except the TBI) and Margalef s, Menhinick's and Camargo's diversity indices appear to be the most sensitive measures for assessing macroinvertebrate response to changes in environmental conditions downstream from the fish farm. In addition, temporal variation in species richness, Margalef s index and biotic indices was similar, exhibiting their highest coefficients of variation at S-4. It is concluded that small waterfalls may improve the streamns self-purification of organic pollution generated by trout farms. © 1992 Kluwer Academic Publishers.