Abstract: It is widely recognized that a colony of Microcystis with several hundreds or even thousands of cells in a field will be replaced by a single cell and a few paired cells after several generations in axenic laboratory cultures. Microcystis aeruginosa were collected from Lake Taihu to conduct a laboratory experiment ((25±1)℃, 2000 lx, 12 h∶12 h) with different levels of ρ(TN) and ρ(TP) of blue-green medium (BG11). The ρ(TN) and ρ(TP) of T1, T2, T3, T4, T5 were 1.00 and 0.05 mg/L, 5.00 and 0.25 mg/L, 25.00 and 1.25 mg/L, 125.00 and 6.25 mg/L, 247.06 and 7.11 mg/L (BG11), respectively. During the experiment, the colony size and the proportion of cells in colonial form of M. aeruginosa increased in T1, T2, and T3, but decreased in T4 and T5. A significant difference was found between the treatments of T1, T2 and T3 and the treatments of T4 and T5 (P<0.05). Colony sizes over than 100 cells were observed in T1, T2, and T3. The colony developed best in T2 and the largest colony was about 960 cells per colony. In contrast, colony formation was entirely absent in T4 and T5. The colony formation of M. aeruginosa absorbed more nutrients and had higher cell growth in low ρ(TN) and ρ(TP). However, high ρ(TN) and ρ(TP) that provide a wealth of nutrients may inhibit or not stimulate the synthesis and secretion of extracellular polysaccharides of M. aeruginosa cells, and this is a disadvantage to colony formation maintainance.