ABSTRACT. – A field study in a region of intensive agriculture was carried out in order to assess the in situ effects of herbicides on natural periphytic diatom communities. Artificial substrates were placed in rivers at ten sites and for five 3-week periods, in spring and in autumn 2008. The composition of diatom communities at the end of each period was related to water chemical conditions. River contamination by herbicides was estimated with passive samplers (POCIS) which can integrate variations in concentrations over a few weeks. Multivariate analysis resulted in the identification of two distinct groups of samples differing by their diatom community compositions and by nutrient and herbicide concentrations. It was difficult to discuss separately the effect of nutrients and herbicides because of the high correlation of these two factors. Diatom communities from the most polluted sites had lower species diversity. Some species of the genera Achnanthidium, Cocconeis and Rhoicosphenia appeared like potential bioindicators of perturbation by agriculture. The observed variations in diatom community composition probably resulted from eutrophication and herbicide toxicity. However in some cases with such investigation methods, the herbicides found in those rivers did not seem to have a prominent effect on diatom communities.