Spatial distribution of picoplankton in the Ross Sea was studied. The authors discuss the biomasses of various picoplanktonic-sized fractions and of bacterial cells between 0.2 and 2.0 μm capable of growing on Marine Agar 2216 (Difco). Picoplankton having a cellular diameter cf between 1.0 and 2.0 μm (PP1) generally predominate, accounting for 73% of the whole picoplankton biomass. However, smaller cells (PP2) can represent 28% of the picoplankton biomass at depths corresponding to 1% of surface light. These results are in good agreement with those found in the coastal regions of McMurdo Sound (Fuhrman and Azam 1980) and in other areas of the Antarctic seas where total bacterioplankton was studied (Hanson et al. 1983b; El-Sayed 1987; Lancelot et al. 1989). Biomasses of total picoplankton (TPP) are not correlated with any of the environmental parameters studied. The PP1 is correlated with O2 and silicates and PP2 is correlated with O2, phosphates temperature and nitrates. Aerobic heterotrophic biomasses are correlated with O2 and salinity.