In May 2010, algal bloom dominated by diatom and blue-green algae broke out in Tangpu Reservoir,which is the drinking water resource of Shao Xing,Zhejiang Province.Phytoplankton samples collected from 3 sites(Kuwei,Kuzhong,and Baqian) along the flow direction were investigated.The phytoplankton density ranged between 1.13×108 and 1.56×108 cells/L.The dominant species Achnanthidium catenatum(Bílý et Marvan) is a pennatae diatom,which was first reported from mainland in China.Another dominant species Pseudanabaena limnetica belongs to filamentous cyanobacteria,and their average relative abundances were 65.6% and 28.2%,respectively.Achnanthidium catenatum is the only planktonic species of the genus Achnanthidium showing distinctive girdle view and colonies consisting of a maximum of 3 frustules.Case studied of three different Achnanthidium catenatum blooms indicated that this species was eurythermal and adapted to lower light intensity and phosphorus concentration.Further study should shed more lights on the formation and disappearance mechanisms of this algal bloom.