The use of herbicides constitutes the principal method of weed control but the introduction of these compounds into the aquatic environment (primarily through runoff) may have severe consequences for non-target plants. In this study, we describe a sensitive and inexpensive method for detection of photosynthesis-inhibiting herbicides, based on chlorophyll (Chl) fluorescence emission. Algae exhibited a Chl fluorescence signature with two maxima around 684 and 735 nm, correlated with the total Chl content of the algal suspension. The ratio of these two maxima (i.e. F684/F735) can be used as an indicator of stress in the photosynthetic apparatus, and thus represents a very simple method for in vivo evaluation of the health status of algae. Determination of the F684/F735 fluorescence ratio revealed the presence and phytotoxicity of atrazine, metribuzin, terbuthylazine, diuron, DCPMU, DCPU and paraquat. The toxic effect of these pollutants was estimated by monitoring the increase in the F684/F735 value, which reflects photosystem II and photosystem I photochemistry. We observed a drastic increase in the magnitude of this ratio, correlating quantitatively with herbicide concentration and corresponding to a decline in algal photosynthetic activity. For the tested herbicides affecting photosynthetic electron transport, the magnitude of the effect was as follows: diuron= DCPMU > metribuzin > atrazine > terbuthylazine > paraquat > DCPU. The F684/F735 Chl fluorescence ratio thus gives toxicity responses which compare favourably with tests such as the algal growth inhibition test, and could therefore be used to detect the presence and phytotoxicity of herbicides in aquatic environments.