Aquatic environments are aften exposed to various pollutants like pesticides due to industrial, agriculture and domestic wastewater discharge. The largest share of pesticides use in the world is related to herbicides, which can be introduced into the aquatic environment through runoff, washing and spraying, and impose adverse effects on their biological and functional properties. Micro-algae have a high value at the basic levels of the food chain in the aquatic environment, and, by absorbing light for photosynthesis, converts co 2 into material that are the main source of food for primary consumers and thus become at the point of beginning of the trophy transmission. In this study, the effect of different concentrations (0, 1, 5 and 10 mg/l) of the metribuzine and chloridazon herbicides on the population dynamics, specific growth, doubling time population, chlorophyll a. b and total carotenoids, absorption rate, alga dry bioma were investigated. The experiment were carried out as completely randomized design for a 14-days at environmental conditions, temperature of 22±1 ?c. photoperiod of 12 hours light: 12 hours dark, and light intensity of 60 ?mol photons/m 2 /s for each set of experiment. The results showed that the effect of herbicide toxicity was different depending on herbicide and its concentrations. In this study, with the exception for the treatment of 1 mg/l of metribuzin, which resulted in a slight increase in cell density, growth, pigments, optical absorption and algal biomass compared to the control treatment, other treatments in both experiments showed significantly decreased in each of the above parameters compared to the control treatment. The lowest amount of mean cell density, specific growth rate, pigments, optical absorption and algal biomass with the highest population doubling time was obtained at the highest concentration (10 mg/l) of metribuzin and chloridazon. In this study showed that the reaction of Isochrysis galbana algae to herbicide is fast, low-cost and easily measurable, therefore, it is recommended for biological monitoring of saline water, especially those exposed to such contaminants. Keywords: Isochrysis galbana , herbicide, metribuzin, chloridazon, growth, biomass.
