The distribution of nano-sized cyanobacteria,which have recently attracted attention of oceanographers for their potential to fix gaseous nitrogen,was extensively investigated during eight cruises in the Pacific Ocean and its adjacent seas using a flow cytometer.Vertical profiles of their abundance at 42 stations show that nano-sized cyanobacteria are confined to the surface mixed layer and their cell concentration at surface reflects that within the whole mixed layer.Extensive survey at surface of 344 sampling stations revealed that nano-sized cyanobacteria distributed at high concentration throughout the North Pacific Tropical Gyre,the western part of the South Pacific Subtropical Gyre and the eastern part of the East China Sea.The concentration was especially high,sometimes exceeding 200 cells mL-1,in the center of the subtropical gyres,around 20 degrees both north and south.These regions with abundant nano-sized cyanobacteria are all characterized by surface temperature higher than 25 °C,high irradiance and nitrate depletion at surface.However,in the Coral Sea,Western Pacific Warm Pool,and the eastern part of the South Pacific Subtropical Gyre,their cell concentrations were lower than 100 cells mL-1,although in these regions surface nitrate levels were extremely low.Low availability of iron or phosphorous in these waters may limit their abundance.The distribution pattern of nano-sized cyanobacteria is in good agreement with the previously reported distribution of N*,which is high in the water column where nitrogen fixation occurs,suggesting their ecological importance as nitrogen fixers.Another notable characteristic of their distribution was patchiness,which was conspicuous near the islands,making it difficult to estimate their total abundance and contribution to nitrogen cycling in the Pacific Ocean.Rough calculation using the present results estimates that nano-sized cyanobacteria in the North Pacific Tropical and South Pacific Subtropical Gyres can contribute to 10-15% of annual nitrogen fixation in the whole Pacific Ocean.In conclusion,the present study revealed ubiquitous occurrence of nano-sized cyanobacteria in the surface water of the subtropical gyres and suggested their significant contribution to nitrogen cycle in the Pacific Ocean,although more intensive survey is required for more credible estimate.