Concentrations of the naturally-ocurring radionuclides 21~
and 2~~
were measured in krill (Euphausia superba), mesozooplankton, phytoplankton and sea water collected during the South African SIBEX cruise to the Antarctic in autumn 1984. The data reported constitute the first substantial measurements on 21~ and 21~ in such samples in the Antarctic Ocean. The concentrations of 2~~ in mesozooplankton and phytoplankton are unexceptional in comparison with those from other oceans. The SIBEX t5. superba, however, have higher levels of 2~~ than usually found in euphausiids. Tile 2~~ data, combined with reasonable estimates of biological quantities such as the fractional assimilation, are used to obtain information about the diet of E. superba. It is suggested that the higher 21~ in the SIBEX E. superba reflects a change from an almost entirely phytoplanktonic diet in summer to a more omnivorous diet as winter approaches. The data show that there are allometric relationships between the 2~~ content of euphausiids and animal size; these are discussed briefly. The limited sea-water data presented are characterized by unusually high 2~ o Po : 21 o Pb activity ratios and need further investigation.