One of the agro-environmental characteristics of Brittany is the extensive use of pesticides, which leads to high concentrations of atrazine in continental water systems. Periphytic algae play a basic part in aquatic ecosystems and some of them, diatoms, are highly sensitive to environmental changes. Analyses of diatoms can thus indicate the impact of a herbicide on aquatic systems. We compared the effects of two herbicides (atrazine and nicosulfuron) on an aquatic ecosystem in 1998. This study was conducted in 15 identical ponds, each containing a holder for installing artificial substrates. The periphytic diatoms were analysed for cell density, biomass, species diversity and morphology. The general specific diversity was poor, and the diatom community was essentially dominated by Achnanthidium minutissimum. The first results showed a strong similarity between the different categories of mesocosms. Chlorophyll a concentration only seems to be affected by high herbicide concentrations.