Despite the recently discovered importance of heterotrophic microorganisms in the structure and function of marine planktonic systems, little is known about the relative significance of these organisms in open-ocean oligotrophic environments. Here we report that depth profiles of planktonic nonphotosynthetic bacteria (BACT), cyanobacteria (CYANO), and both photosynthetic (PNAN) and heterotrophic (HNAN) nanoplankton (i.e. eukaryotes C 2 0 pm diam.) to 2600 m in the Sargasso Sea in summer and fall (the least productive times of year) indicate that BACT biomass dominates these microorganisms at all depths. From the microscopic counts and our 'best guess' conversion factors, we determined that BACT contain >70 and >80 '5, of the euphotic zone microbial C and N, respectively, and >90 YO of the biological surface area, C Y A N O , P N A N , and HNAN b~ovolume each make up 7 to 17 % of the total. Even calculations based on maximum probable phytoplankton biomass (from chlorophyll) and minimum probable BACT biomass still showed domination by BACT in the euphotic zone. An important practical implication is that conventional measurements of particulate organic C and N collected on glass fiber filters may be missing most of the biomass. Independent bacterial heterotrophic production estimates based on tritiated thymidine incorporation and clearance by nanoplanktonic grazers suggest bacterial division rates of about once per 5 to 15 d. Interpretation of these results suggest that BACT consume significant amounts of C in this system, probably released from phytoplankton dlrectly or via herbivores. These results imply a fundamental difference between oligotrophic and eutrophic waters with respect to biomass distribution and that BACT are major particulate reservoirs of C and N in the sea.