Iron, copper and zinc at toxic levels were found in water and sediment structure of Kucukcekmece Lake. In the research area, algae blooms have been observed dependent upon anthropogenic and abiotic factors at certain periods. Cyanobacteria species have been observed as dominant microorganism during algae bloom. The objective of the present study is to evaluate the toxic effects of various heavy metals on the Synechocystis sp. E35 isolated from Kucukcekmece Lagoon, Istanbul. Toxicity of Zn⁺², Fe⁺², Cu⁺² and of metal mixture (Zn⁺²- Cu⁺², Zn⁺²-Fe⁺², Fe⁺²-Cu⁺² and Fe⁺²-Cu⁺²-Zn⁺²) are investigated in the laboratory scale. The order of toxicity of the metals to the Synechocystis sp. E35 is Cu⁺²> Zn⁺²>Fe⁺² in growth media. The EC₅₀
values of the Zn⁺², Fe⁺², Cu⁺² on the inhibition of the growth of E35 isolate: are 6.94, 13.92 and 3.42 mg/l, respectively. Besides, combination of ternary metal systems showed stronger inhibitory effect on cells are 0.02 and 2.93 mg/l, respectively.