A team from Cemagref in Lyon (FR) has identified a pertinent indicator of the level of pollution in rivers: the biofilm. A complex combination of bacteria, algae and fungi, the biofilm forms a viscous layer on immersed supports, such as the stones on a river bed. The biofilms are able to break down the pesticides that contaminate aquatic environments and modify their structure, diversity and functioning. Toxic substances, for example, can change the photosynthetic, respiratory or enzymatic activity of aquatic ecosystems, either definitively or temporarily. It is in this way that biofilms can serve as an early warning signal of pollution. It remains to characterize and distinguish the responses of biofilms to pollutants from those induced by environmental factors (current speed, physico-chemical composition, etc.). It is research of this kind that should help respond to the need set out in the Water Framework Directive to improve the ecological quality of European rivers.