The protection of streams from pollution by pesticides is currently among the highest environmental priorities. Despite this, the majority of bioindicators used in monitoring programs do not specifically target this issue. Among the battery of such bioindicators, benthic diatoms are used to demonstrate the impact of nutrient pollution in streams. Given the variable sensitivity of their species to chemical pollutants, benthic diatoms could also be excellent indicators of pesticide pollution. In this project we aim to identify benthic diatom taxa that are specifically sensitive or tolerant to pesticide pollution. In order to achieve this objective, the project is structured around three interconnected research axes: (1) laboratory experiments under controlled conditions that will distinguish the effects of pesticides from those of a set of important environmental factors such as temperature, light, and nutrients; (2) field study campaigns to relate (and validate) the results obtained from the laboratory experiments to real environmental conditions; (3) more detailed analyses of existing Swiss databases that have been generated by the different monitoring programs. These analyses will also be based on the results obtained in the laboratory and field experiments and via mathematical modeling approaches. Eventually, the data obtained in the project will be used to develop a diatom index that is specific to pesticides and that can be implemented in Swiss river biomonitoring programs, along with the other indicators.