In order to investigate the structure of phytoplankton communities and chlorophyll a, this study was carried out at 9 stations from June 21 to June 26, 2006 in the Bering Sea. Concentrations of total microphytoplankton and nano-picophytoplankton chlorophyll a were similar in the eastern shelf. But in the Aleutian Basin, concentrations of total microphytoplankton were higher than nano-picophytoplankton chlorophyll a. Chlorophyll maximum depth were approximately 20 meter in the eastern shelf and surface in the Aleutian Basin. Phytoplankton dominant species were Thalassiosira sp., Chaetoceros sp. and nano-picophytoplankton such as Dinobryon belgica. Phytoplankton communities were composed of 48 taxa representing dinoflagellate, diatoms, cryptophyceae, chrysophyceae, dictyochophyceae, prymneosiophyceae. The most abundant species was picophytoplankton in all station except station 4, but the second was variable. Phytoplankton standing crops ranged from minimum 9.8×105cells ℓ-1 at station 137 to maximum 2.0×107cells ℓ-1 at station 135 in surface and 8.7×105 cells ℓ-1 at station 138 to maximum 4.1×106 cells ℓ-1 at station 151 in SCM depth. 1) To investigate on species composition, abundance and dominant species of phytoplankton communities in the Bering Sea 2) To study on taxonomic research and dominant species of phytoplankton communities for investigate on indicator species