A strain of small-sized dinoflagellates,isolated from the culture of sediment incubation collected from the coastal areas in south China,has been identified under microscopical observation and rDNA sequence.Surface sediments from two sea areas in the south Chinese coastal waters were incubated for 20 and 40 days,and germinated vegetative cells were observed.The cells were identified as species in the Suessiaceae based on the morphological characteristics,ultrastructural features of the cell,as well as its swimming behavior.The studied strain clusters into a well-supported clade together with six sequences of Biecheleriopsis adriatica in the phylogenetic tree based on the large subunit (LSU) rDNA sequence.Therefore,the strain has been identified as B.adriatica based on morphological observation and phylogenetic analysis.B.adriatica was the dominant dinoflagellate species in the germinated phytoplankton community from both sea areas,which contributed 50％-83％ to the total germinated dinoflagellates averagely.However,B.adriatica has not been reported in previous phytoplankton surveys,and was probably ignored or misidentified due to its small size and thin wall.The frequent and abundant occurrence of B.adriatica in the germinated phytoplankton community of many sea areas of the south Chinese coastal waters suggests its wide and abundant distribution in these sea areas.