Abstract: To support the basic data for forecast of algal blooms and ecological research of Microcystis,circadian vertical migration experiment was carried out at a fixing station bimonthly in Lake Erhai from November 2009 to September 2010.The results were as follows:The highest density (1.10±0.52)×107 ind/L of Microcystis was found in November 2009 and the lowest value (2.06±1.05)×105 ind/L occurred in March 2010.During water blooms (high density) period,Microcystis colonies in all months exhibited strong diel vertical migration (DVM) behavior.Due to the influence of light illumination,Microcystis stayed in deeper strata during the daytime and migrated into the epilimnion at night and before dawn.The distribution of Microcystis was weakly related to temperature in July (r2=0.367).Microcystis colonies had significant correlation with pH in July and September (P<0.001),and the correlation coefficients were 0.823 and 0.712 respectively.Microcystis also showed strong diel vertical migration in low density (except for May 2010),among which,the migration pattern in January was similar with that in water blooms (high density).The aggregation pattern of Microcystis was found in March which may be associated with predation of Daphnia.