The use of Rapid light curves (RLCs) as a toxicity endpoint for river biofilms was examined in this studyand compared to "classical fluorescence parameters" i.e. minimal fluorescence (F0), optimal and effectivequantum yields of photosystem II (Fv/Fmand PSII). Measurements were performed after exposure tofive concentrations of diuron (from 0.3 to 33.4 g L-1), its main degradation product (DCPMU) (from1.0 to 1014 g L-1) and norflurazon (from 0.6 to 585 g L-1) with the lowest exposure concentrationscorresponding to levels regularly encountered in chronically contaminated sites. Biofilm responses wereevaluated after 1, 5, 7 and 14 days of exposure to the different toxicants.Overall, the responses of both "classical fluorescence parameters" and RLC endpoints were highly timedependent and related to the mode of action of the different compounds. Interestingly, parameters cal-culated from RLCs (ETRmaxand Ik) were useful early markers of pesticide exposure since they revealedsignificant effects of all the tested toxicants from the first day of exposure. In comparison, classicalfluorescence endpoints (F0and Fv/Fm) measured at day 1 were only affected in the DCPMU treatment.Our results demonstrated the interest of RLCs as early markers of toxicant exposure particularly whenworking with toxicants with less specific mode of action than PSII inhibitors.