Abstract : The abundance, biomass and distribution of phytoplankton, bacterioplankton and heterotrophicprotists in the Bering Sea were investigated from July to August 1999. Chlorophyll a concentrations in thesurface waters ranged from 0.16 to 3.79 µg l −1 . Nano-phytoplankton were found to constitute from 63 to98% of the total phytoplankton biomass, and were clearly the dominant primary producers. The biomass ofbacterioplankton in the surface layers varied from 1.46 to 20.2 µg C l −1 and accounted for 30% of the totalphytoplankton biomass. The biomass of bacterioplankton integrated over a depth of 0 to 100 m averaged65.4% of the total phytoplankton biomass. The surface biomass of heterotrophic protists ranged from 1.2 to27.4 µg C l −1 , and was within the same order of magnitude as that of bacterioplankton. Of the total biomassof heterotrophic protists in the upper 100 m of the water column, 65% was attributed to protists in thenano-size class. The results of this study suggest that bacteria and nano-protists are important componentsof the planktonic community in the Bering Sea during the summer season. The abundance of bacteri-oplankton and planktonic protists decreased from the western to northeastern and eastern regions of theBering Sea. The abundance of these organisms also decreased with depth. The available evidence suggeststhat variation in the abundance and distribution of these organisms may be affected by water currents andvertical temperature variation in the Bering Sea.Key words : bacteria, protists, biomass, ecology, Bering Sea