In order to ascertain the different tolerances to chemical contaminants in one strain of Microcystis with different morphology, unicellular and colonial Microcystis in one strain was obtained from different conditions of light intensity and temperature. The samples were divided into 8 groups including control (no chemical addition), CuSO4, chloromycetin, and linear alkylbenzene sulfonatelas (LAS) treatments. The cell density, cell viability, superoxide dismutase (SOD), and malonaldehyde of Microcystis were analyzed. It was observed that cell density of both unicellular and colonial Microcystis increased from the beginning to day-5 in the control and the CuSO4 treatments. However, the growth of Microcystis was significantly inhibited in the culture with chloromycetin and LAS treatments. Notably, the inhibition rate was significantly high in unicellular Microcystis relative to the colonial Microcystis. The esterase activity in all the treatments decreased dramatically relating to the control. In addition, the esterase activity in colonial Micro cystis was significantly higher than that of the unicellular Micro cystis in all the treatments. Although there were no significant differences in activities of SOD between the two morphologies in the control treatments, in all the other treatments, significant differences were observed. The results proved that colony formation of Microcystis could be considered as a strategy in response to chemical stress. (C) 2014 Elsevier B.V. All rights reserved.