The primary role of pesticides is to control insects, fungi and other parasites on agricultural cultures, but because of their unspecific mechanism of action, they often negatively affect other organisms. Single celled protists such as Euglena viridis, play an important role in transfer of energy and nutrients within food webs of aquatic ecosystems, so it is important to know their population structure to keep the ecosystem stable. Studies have shown that imazalil has a neurotoxic and teratogenic effect on rodents, is toxic to aquatic invertebrates and vertebrates, but there is not any specific research on freshwater single celled organisms. The aim of this paper was to simulate the natural conditions in the laboratory to investigate the potential toxic effects of pesticide imazalil on freshwater organisms using the E. viridis as a model organism. The influence of pesticides on the survival and population dynamics of the species and the biochemical changes occurring within the investigated organism were studied in the treatment of different pesticide doses. LC50 concentration of pesticides was measured as 180 μM. At low concentrations of the pesticide, changes in the cellular movement occur, 50% of the organisms in the population become immobile. At doses higher than 500 μM, changes in cell viability occur. Imazalil inhibits the MTS enzyme reaction by 60% but does not affect the activity of LDH enzyme. Metabolic disorders are indirectly monitored and follow the changes in the intracellular redox balance of biochemical reactions. Within cells at a dose of 200 μM, intense formation of intracellular hydrogen peroxide results in increased activity of the SOD enzyme.