Abstract: The dinoflagellate Peridiniopsis was one of bloom-forming species during the spring period in most bays of Three Gorges Reservoir(TGR).Account of diel vertical migrations may be of fundamental importance in understanding the ecology of dinoflagellates and explaining their dominance in bloom.The bloom-forming species Peridiniopsis with the characteristic of vegetative cells free-swimming,should exhi-bit active vertical movements in water column.However,there was no such information in previous literatures.In the present study,diel vertical distribution patterns of phytoplankton were examined from Feb.26 to 27,2007,when the bloom badly occurred in Xiangxi Bay of the TGR.The dominant species Peridiniopsis sp.3 accounted for over 45% of total abundance.Patterns of diel vertical distribution were elucidated quantitatively using two different analysis methods.Firstly,mapping method was used to describe the time-depth distribution of phytoplankton individuals and abiotic factors.Secondly,the significant differences of vertical distribution from a state of randomness were tested with Morisita’s index.For the dominant species Peridiniopsis sp.3,chlorophyll a and total phytoplankton,the degree of aggregation showed an aggregated distribution in the water upper layer by day and was close to a random distribution in the water column by night.Correlation analysis showed that the diel distribution patterns were affected by photosynthetically available radiation(PAR,400-700 nm).The distribution patterns of phytoplankton and Peridiniopsis were coincident with that of PAR.