Paraquat toxic action on the freshwater microalga Chlamydomonas moewusii was determined by several physiological parameters. ATP content seems to be the most sensitive parameter analyzed; it was reduced by the lowest paraquat concentration assayed (0,037 µM), because of the high energy consumption provoked by the stress. Nevertheless, a paraquat concentration 0,30 µM was necessary to alter microalgal growth significantly. Photosynthesis rate and chlorophyll a fluorescence were also reduced by a paraquat concentration lower than 0,30 µM. HPLC chromatograms of photosynthetic pigments showed also the paraquat toxicity on C. moewusii, since in chromatograms of cultures with 0,30 and 0,60 µM of this herbicide the peak of ß-carotene was not detected. Our results show the necessary use of microalgae in ecotoxicity studies; measurements of new sensitive physiological parameters contribute to improve earlier detection of damages in the first level of aquatic food chain provoked by low concentrations of pesticides.