A test was developed that measures in vivo chlorophyll a fluorescence variables to assess the apparent sensitivity of freshwater periphytic algae to photosystem II inhibitors. Natural periphytic communities from rivers were collected on artificial substrata, and the effects of short-term exposures to two PSII herbicides (atrazine andisoproturon) on the fluorescence parameters were measured with apulse-amplitude modulated fluorometer. The EC50 for each herbicide were calculated from fluorescence yield indices, and these results were compared to 14C-based primary production measurements on the same communities. The fluorescence-based method appears to give very reliable estimations of EC50 for each pesticide we tested, ranging from 0.46 to5.18 µM and 0.07 to 6.77 µM for a trazine and isoproturon,respectively. This method could be used in ecotoxicology monitoring programs, to detect changes in natural periphyton populations sensitivity,following photosystem II herbicide contamination in rivers or lakes.