Benthic diatoms are routinely used to assess river pollution. Most of the tools are using pollution sensitivities of diatom species. Determination of diatom species requires a long and regular practice because its flora is extremely diverse. Moreover morphological features to separate numerous taxa are scarce. The choice of taxonomic resolution is very rarely asked for diatom bioassessment. The objective of this study was to test on a database the impact of taxonomic resolution on information loss. Our final datasets, gathering about 2000 diatom samplings carried out in the framework of Biomonitoring rivers in the Rhone-Mediterranean and Rhine-Meuse basins, combined 3 kinds of tables: (a) taxa abundance by site arrays on the species, genus, family, order, class and sub-division; (b) physical and chemical site characterization; (c) site typology. Based on Mantel tests, it appeared that all the arrays were significantly correlated (except for subdivision). They were even very highly correlated from species to class resolutions. Canonical correspondence analyses showed that based on chemical parameters (nutrient, organic matter and mineralization) the highest percentage of variability in the diatom community matrix that could potentially be explained was met for the order level. The poorest were met for sub-division and species resolutions. Finally, based on Corrected Rand Indexes, we observed that the more precise the taxonomic resolution we have the more correlated to the hydro-ecoregional classification we got. These results lead to wonder if a very fine taxonomic resolution is necessary to assess classical pollution (nutrient, organic matter). Hypotheses to explain such results are that (1) many species are too rare to establish their ecological requirements with certainty (2) more environmental descriptors are necessary to explain presence of some species (3) the dataset is surely marred by identification errors especially at species level. On the other hand it seems important to keep a species level identification to know the ecoregional membership of the diatom community.