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We investigated riparian ground beetle communities to: (1) identify the main environmental factors determining species composition; (2) determine whether sites with similar ecological characteristics have similar species composition as a result of local selection; and (3) provide some indications for conservation of these insects. For 45 sampling sites in the River Aniene we measured: elevation, river bank morphology, percentage of shaded area, habitat heterogeneity, sediment depth, sediment granulometry and geographic position. We then used Canonical Correspondence Analysis (CCA) to investigate the importance of these variables in structuring beetle communities and cluster analysis (CA) to identify groups of sites with similar species composition. Granulometry and elevation were the most important parameters influencing species composition. Position of assemblages in CCA space was partially similar to their groupings in CA. In particular, silty sediments hosted a homogeneous group of species (with several highly specialized elements), well separated from all other clusters, suggesting that this type of substrate exerts a strong local selection pressure on ground beetle species. Communities characterized by species associated with coarse sediments were typically found in isolated localities. Further isolation of these populations by impoundments would have a serious impact on their persistence. [ABSTRACT FROM PUBLISHER]