The diurnal migration of Microcystis was studied using a cylindrical reactor to demonstrate how colony morphology affects the tendency of Microcystis cells to float or sink.Experiments were carried out using single cells and colonies from laboratory incubation and also colonies from Lake Taihu.Results showed that single cells had significant diurnal migration while this was apparently insignificant in colonies either from laboratory incubation or natural water.In contrast,colonies taken from Lake Taihu exhibited remarkable floating features.These were observed to be due to differences in specific gravity between cells from laboratory incubation and Lake Taihu.In addition,colony morphology affects significantly the upward movement of the cells.