Abstract: A large-area cyanobacterial bloom was observed for the first time in some bays of Three Gorges Reservoir(TGR) in June 2008,which indicated the deterioration of the aquatic ecosystem.Thick blooms covered most parts of Xiangxi Bay that is a classical bay of TGR on 20 June 2008.Fourteen sampling sites were monitored weekly from 6 June to 25 July 2008,to study the dynamics of the algal bloom in the bay.The whole process of this algal bloom and some influencing factors were analyzed in this paper.The results showed that the bloom broke out on about 9 June 2008;and reached the summit on 20 June and ended on 11 July,lasting for about a month.According to Carlson's Trophic State Index(TSI) criteria,Chl.a content is equal to 20 μg/L and TSI is 60 are the critical values of cyanobacterial predominance.In the present study,the intersection of Chl.a content is more than 20 μg/L and TSI is more than 60 were set as the threshold state of the cyanobacterial bloom.According to the criteria,the whole process of the bloom could be divided into 2 stages:9-25 June was the first stage and 3-11 July was the second stage.The zone of the algal bloom was mainly concentrated in the region about 20-30 km away from the river mouth,namely the middle-upper reaches of the bay.This indicated that suitable hydrodynamic conditions were necessary to the outbreak of the cyanobacterial bloom.Low phosphorus and high N/P were the main factors limiting the bloom.When phosphorus content decreased and N/P increased significantly,the growth of the cyanobacteria was suppressed.The results also preliminarily indicated that N/P within the range between 10:1 and 25:1 was more favorable to the growth of the cyanobacteria in this area.So,decreasing the input of the phosphorus is of great importance to prevent or control the algal blooms in Xiangxi Bay.