A key issue in the implementation of the Water Framework Directive is the classification of streams and rivers using biologicalquality parameters and type-specific reference conditions. Four groups of stream types were defined in NE Spain on the basisof 152 diatom samples by means of detrended correspondence analysis and classification techniques. Diatom analysis was restrictedto epilithic taxa, and the sites included gradients ranging from near-natural streams to sites with poor ecological quality.The main gradient shows a clear separation of sites in relation to the degree of human influence: polluted streams (mainly locatedin the lowlands) differ from streams in mountainous areas and in the Pyrenees. A second gradient is related to physiographicalfeatures. Headwater streams can be distinguished by their catchment geology. The type-specific diatom taxa for the streamtypes studied were determined by using indicator species analysis (IndVal). The type-specific taxa from near-natural streams arecoincident with the indicator taxa for high ecological status. Human impact reduced the typological heterogeneity of the diatomcommunity composition. Overall, the diatom communities in NE Spain exhibit a regional distribution pattern that closely correspondswith that observed in river systems elsewhere. Physiographical differences are only evident in undisturbed sites, whilenutrient enrichment and other human disturbances may mask the regional differences in the distribution of diatom communities.