Description: Abstract - Flow cytometry and epifluorescence microscopy were employed to determine the abundance, distribution and size structure of autotrophic pico- and nanoplankton on a series of transects along 6°W between the Polar Front and the receding ice-edge during 12 October-21 November 1992. Abundances ranged from〈1.0 to 〉20.0x10 6 cells 1 -1 in the upper 100-m layer. Lowest surface concentrations were found at the receding ice-edge between 56° and 59°30'S. Abundances increased with advancing season within the Polar Front region(47°-50°30'S) and in the Southern Antarctic Circumpolar Current zone (ACC, 51°-54°30'S) but not south of the ACC-Weddell Gyre Boundary (54°30'-58 °30'S). Autotrophic pico- and nanoplankton contributed up to 90% to total chlorophyll a in regions with low chlorophyll a concentrations (0.2-0.4 µg 1 -1) but less than 50% in regions were phytoplankton biomass accumulated (〉1.8 µg chl1 -1, Polar Front region), indicating a shift in dominance towards larger size classes in areas of high chlorophyll a concentrations. Referring to cell numbers, this shift towards larger size fractions was observed within the autotrophic pico- and nanoplankton. The build up of phytoplankton biomass to bloom levels was primarily due to the accumulation of diatoms. Hence, regional variability in magnitude of the spring bloom is due to diatoms while the standing stock of autotrophic pico- and nanoplankton is less variable. We hypothesize that neither micronutients (e.g. Fe) nor grazing suppresses abundance of pico- and nanophytoplankton below a "background" concentration of about 2.0-4.0x10 6 cells 1 -1 during spring and early summer in wide areas of the Southern Ocean. Our observations confirm hypothesis that both groups represent abundant and stable constituents of the phytoplankton assemblage in the Southern Ocean.