For many years, studies have shown a clear degradation of the water quality of European rivers. Among the potential contributors to this evolution, agricultural practices have been identified as the main source of pollutants. Many bio-monitoring tools like the diatom indexes (IBD, IPS, )exist to reveal water pollution by nitrate and phosphate. Nevertheless, they are mainly focused on trophic issues and are not proved to be efficient to detect the effects of pesticides pollutions on stream diatom flora. In the face of the European Directive (2000-60), water resources management agencies require new bio-monitoring tools able to reveal and to assess agricultural toxin pollutions. In this context, Cemagref has launched a research program to identify the main impacts of pesticides on diatoms communities, to understand the biological mechanisms implied in the reactions of diatoms in order to select the most efficient descriptors and improve the bio-monitoring of pesticides water pollution. In a first step, a field study have been conducted in 2005 and 2006 in rivers draining a intensive agricultural area in the South-West of France, aiming to study the impacts of agricultural pollution on natural diatoms communities and to select few biological descriptors of this pollution. Secondly, to better understand and test the cause-and-effect relationships between herbicides exposure and biological parameters evolution, an artificial streams study has been conducted under controlled experimental conditions. The purpose of this work is to ensure the efficiency of more accurate descriptors that reveal specific water pollution effects of pesticides on diatom communities.