We sampled pupal exuviae in 39 Mediterranean stream sites, situated in 2 mountain regions in the Betic ranges (southern Spain) to test the value of chironomid species assemblages for stream classification and water-quality evaluation. Downstream reaches of these streams are organically polluted to different extents, but the headwaters are protected, providing reference sites for comparison with impaired sites in the mid elevations. A priori classification of sites was done using 7 physiographical and geological variables to differentiate stream types independently of human influence. A posteriori classification and ordination analyses of sites were done on an environmental matrix of 28 variables and a biotic matrix of 204 chironomid species. Regional-scale factors played a more important role than reach-scale factors (i.e., water chemistry) in structuring chironomid assemblages. However, within each region, reach-scale factors such as thermal regime and variables related to water-quality impairment also exerted important influences on chironomid assemblage structure. The indicator value of chironomid species, obtained by comparing reference and impaired sites in the same stream type, showed that 6 species clearly had a positive response to water-quality impairment. This response does not appear to be dependent upon stream type or regional setting, either in the studied area or in other fluvial ecosystems of Western Europe. Moreover, these chironomid species seemed to be distributed along a wide gradient of water-quality impairment, suggesting their potential for evaluating a great range of water-quality degradation.