Two algal whole cells biosensors are developed to measure specific toxicity of freshwater pollutants. Both optical and conductometric biosensors are based on inhibition of algal alkaline phosphatase (AP) and esterase activities. Chlorella vulgaris cells are immobilised on a membrane placed in front of an optical fiber bundle for optical sensing or deposited on the surface of an electrode for conductometric sensing. Phosphatase activity of the biosensor is strongly inhibited by heavy metal ions (60% loss of activity is obtained after 10 ppb Cd2+ and Zn2+ with 20 min exposure time), as equally observed with a microplate reader. Inhibition of esterase activity (EA) is actually achieved with organophosphorous pesticides such as methyl paraoxon. The biosensors exhibit a response time of about 5 min. These pollutants can be detected down to 10 ppb after being in contact with the biosensor for 30 min. The biosensor can be used up to 20 days with 90% remaining activity.