1-We examined the accuracy of a multimetric approach based on 22 biological and ecological traits of benthic macroinvertebrate communities to assess toxic sediment pollution in streams. 2-Faunal and chemical data from sites located on 150 medium-sized mountain streams in France were analysed. We used eighteen additional sites, not included in the multimetric tool development as a test data set. A toxic quality class (from high' to poor/bad') was pre-assigned to each site using the French water quality assessment system (SEQ-Eau) based on toxic substances (metals, PAH, PCB) in sediment. 3-Each trait was described in multiple categories. The affinity of macroinvertebrate taxa for the different categories of a trait was described using a fuzzy coding procedure. The relative abundance of trait categories was calculated for communities at each site. 4-A non-parametric multiple comparison statistical procedure was used to compare relative abundances of trait categories between groups of sites assigned to different quality classes (e.g. high vs. good; good vs. moderate, etc.), to identify the combinations of trait categories that best separated sites between adjacent toxic quality classes. Based on such sets of trait categories, we propose a statistical procedure to allocate sites to toxic quality classes from the attributes of its benthic macroinvertebrate community. 5-Predictions from the trait-based functional tool achieved approximately 73% of correct site post-assignments to toxic quality classes pre-assigned using chemical criteria. All of the sites pre-assigned to the 'high' quality class, were post-classified into the same quality class using benthic community attributes. From 0-90 % of sites from other quality classes were classified identically using both chemistry- and trait-based criteria, the observed biological impact being often lower than expected. We suggest that the rapid decline of sensitive species could lead to both low taxonomic richness and low diversity of adaptations, giving communities a less diverse range of potential functional responses to increasing toxic pressure. 6-Biological criteria based on macroinvertebrate traits could provide new methods for biological assessment. This study is the first step towards an in situ functional tool of stream sediment contamination assessment at community level. However, we need to increase the number of sites included in the development data set to improve the tool's precision, extend the procedure to additional stream types and validate the design at a larger spatial scale.