Metal pollution is considered hazardous to biological systems because of their toxic and carcinogenic potential. The behavioral, morphological and physiological changes have been prominently used in toxicity assessment studies in unicellular organisms. Recent studies have demonstrated that ciliates and flagellates play an important trophic role in periphytic communities and as bioindicator organisms for pollution degree in aquatic ecosystems. The present research was carried out to assess the

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... rrelation of toxic stress and deleterious impact of heavy metals (Copper, Cadmium, Zinc and Lead) on population densities of fresh water protozoans. Toxicity of four metals Cu 2+ (CuSO 4 . 5H 2 O), Cd 2+ (CdCl 2 ), Zn 2+ (ZnSO 4 ) and Pb 2+ (Pb(NO 3 ) 2 ) were studied at three different concentrations 50μg/ L, 100 μg/ L and 150 μg/ L each. Direct count method was employed for enumerating the population densities and toxic stress responses were seen quantitatively as well as qualitatively. Colonization rates expressed by the species number over time obtained depicted a geometric regression of the average number of protozoans with increasing concentration of metals as compared with controls. Irregular beating of flagella and cilia and erratic and spinning movements of ciliates along with blebbing, stress egestions were observed in presence of these toxic metals. Copper was found to be least toxic followed by Cadmium, Zinc and Lead in terms of increasing toxicity.Pb and Cd form organometallic complexes, the toxicological properties of which can be more toxic than individual ions due to their high bioaccumulation tendencies. With significant metal uptake tendencies, protozoans can have a potential use in bioremediation of polluted waters.