Diatoms are commonly used as bioindicators of trophic and saprobic pollution in rivers. However more knowledge is needed on their sensitivity to toxicants such as agricultural herbicides, to use them as indicators of environmental contamination. Several species of periphytic diatoms were isolated from the Morcille River which presents a contamination gradient by herbicides and particularly diuron. Sensitivity of these species to diuron was assessed through ecotoxicological tests based on short term growth inhibition of monospecific cultures. Surprisingly growth stimulation was observed for each species at low herbicide concentration and toxic effect was visible in average from a concentration of 10 µg.L-1. After application of an appropriate hormesis model, Effect Concentrations (EC50) were determined and the species were ranked according to their tolerance. Finally the results were related to field community samples from an upstream and a downstream station in order to check if variations in specific relative abundance between stations are consistent with differences in tolerance to diuron.