ABSTRACT. – The objective of this study was to identify the response of aquatic Coleoptera to water quality and habitat variables in two rivers in northwest Spain with different degrees of contamination. The water beetle assemblages of the Lagares and Miñor Rivers were studied during an annual cycle (May 2001-January 2002). Several physical and chemical variables, in addition to habitat and spatial factors were measured at each site. The environmental variables were then subjected to PCA to facilitate the interpretation of the principal components. The structure of the assemblage was evaluated for different richness measures: Species richness rarefied (ES); Abundance (N) AND Shannon-Wiener Diversity Index (H’). Regression analysis was used to examine the relationships between the richness measures and the water quality variables and the relationship between species and environmental variables was explored by Canonical Correspondence Analysis (CCA). Differences in assemblage structure were examined using analysis of similarities with Bray-Curtis index. We found a total of 35 species of Coleoptera belonging to 10 families, 37 % of which are endemic to the Iberian Peninsula. All richness measures decrease in degraded sites, with a negative correlation between these factors and sites with high levels of contamination. Data analyses also indicated significant differences among the assemblages of non-polluted and polluted waters. CCA showed that most of the species seemed to be sensitive to pollution, only Limnius volckmari (Panzer) and Elmis aenea (Müller) might be considered tolerant species.