9 6-h bioassay experiments were conducted in order to investigate the influence of the herbicide Ametryn on growth and metabolic activities of three freshwater  microalgae, namely Scenedesmus quadricauda (Trup) de Brebisson, Staurastrumnatator (W.west) (Chlorophyceae) and Microsystis flos-aquea (wittr) Elenkin (Cyanophyceae). The EC50 values of the herbicide Ametryn for inhibiting growth,chlorophyll a dry weight, carbohydrate lipids and protein were determined. The three tested algae and their tested parameters varied greatly in their response to  metryn. Scenedesmus quadricauda was the most susceptible species to Ametryn while - aquae was the most tolerant. Lipid proved to be the tolerant parameter to Ametryn in comparison with the other physiological parameters