This work compared the sensitivity of three cyanobacteria (Anabaena flos-aquae, Microcystis flosaquae and Mirocystis aeruginosa) as well as five green algae (Pseudokirchneriella subcapitata, Scenedesmus quadricauda, Scenedesmus obliquus, Chlorella vulgaris and Chlorella pyrenoidosa) to four pesticides through 96h short-term chronic tests. The results showed that the toxicity of the pesticides to the organisms increased in the order: propiconazol > isoprocarb > flumetralin > propargite. A wide variation in toxicity response of the organisms was observed. The sensitivity of the organisms varied by over one order of magnitude for propargite, by over two orders of magnitude for isoprocarb and propiconazol, and by over three orders of maginitude for flumetralin. Compared to green algae, cyanobacteria were less sensitive. This may result in the alteration of green algae dominated species to those dominated by cyanobacteria, stimulating to cyanobcterial bloom during a certain period.