During the icebreaker SHIRASE cruise of the 27th Japanese Antarctic Research Expedition (JARE-27; 1985/86), vertical profiles of phytoplankton chlorophyll a concentration in the upper 200m of the water column were observed at 12 stations in the Southern Ocean and at 3 stations in the subtropical water from December 1985 to March 1986. High phytoplankton chlorophyll standing crops (ca. 370mg m^<-2> in December and 330mg m^<-2> in February) were observed in Breid Bay, Antarctica. In other stations of the Antarctic Ocean, the standing crops were less than 52mg m^<-2>. Size fractionation studies revealed that net-phytoplankton (>20μm) was the dominant fraction of total chlorophyll a during the summer bloom in Breid Bay. In Antarctic waters, the high contributions of the net-phytoplankton fraction corresponded to high total biomass. And also, contribution of the net-phytoplankton to the total phytoplankton was supposed to be controlled by the length of the ice-free period. Considering the high phytoplankton growth rates under the nutrient rich condition, duration under optimum light condition and water stability appear to be important factors affecting the phytoplankton crops in the Antarctic Ocean in summer.