Abstract: Microcystis has been recognized as the dominant bloom-forming organism in eutrophic lakes world-widely. Its dominance in natural circumstances results from a variety of adaptive strategies developed by Microcystis of which the essence is to exist in the form of colonies. The existence and size of Microcystis colonies change in response to the cyclical rhythms of key factors such as temperature and light. For example, summer is a preferable season for Microcystis to form colonies in large sizes, while in winter and early spring the organism are more readily to maintain a form of either single cells or small colonies. To understand how light regulates the morphology of colonial Microcystis, we selected six different species/strains (TH-M2, DC-M1, FACHB1174, FACHB1027, DH-M1 and DC-M2) of colonial Microcystis and compared their responses to varied irradiance, in terms of colony size, growth, content of EPS and microcystins. It showed that the colony sizes of all Microcystis were dramatically enhanced by the increased light intensity. When the light intensity was in the range of 80200 mol/(m2s), the enlargement was caused by the significant increase in EPS production in high light saturation point strains TH-M2, DC-M1, FACHB1174 and FACHB1027. However, the enlargement was caused by the increased growth rate in low light saturation point strains DH-M1 and DC-M2. This implied that the mechanisms inducing the enlargement of colony size at high light intensities were diverse among different species/strains of Microcystis. In high light saturation point strains the enhanced production of EPS played an important role in forming larger colonies, probably via the strengthened adhesion of single cells; whereas in high light saturation point strains the increase in colony size is most likely due to a faster growth rate. We also examined the transcriptional levels of mcy genes and concentrations of intracellular microcystins. It was observed that at high light intensity the expression of mcyB and mcyD were enhanced in all toxic Microcystis, and microcystins content was increased by the elevated light intensity. These results indicated that microcystins might play a role in colony formation and maintenance. Our study illustrated the correlation between the size of microcytstis colonies and the light. The data suggested that the enlargement of colonial Microcystis is the outcome of the physiological adaptation to light intensity through various mechanisms in different species/strains.