Size fractionated phytoplankton chlorophyll a (>10 μm and <10 μm) were determined for the waters in the surface mixed layer of the offshore areas around Japan in spring and summer. The chlorophyll a concentration of > 10 μm fraction largely varied with the sampling locations ranging from almost zero to 5.3 μg l-1. On the other hand, the chlorophyll a concentration of <10 μm fraction showed relatively narrow range of variation being roughly lower than 0.5 μg l-1. The obtained results provided the evidence of the ubiquitous occurrence of small-sized phytoplankton in the offshore waters around Japan as reported in other regions, while the abundance of large-sized ones can be affected by the status of the environmental conditions.