The waters in the vicinity of the South Shetland Islands and Elephant Island were surveyed to investigate mechanisms for the formation of krill concentrations and to estimate acoustic biomass of krill in the 1990/91 austral summer. Main krill concentrations occurred in the shelf waters north of the islands, as usual. Tracks of drifting buoys (with curtain drogues at 30 m depth where krill frequently occur), have elucidated the occurrence of convergent complex eddies in these shelf waters. (Note is that one buoy traveled all the way to South Georgia and has been being trapped in a winter fishing ground for krill.) High concentrations of chlorophyll a were also measured in the shelf waters, showing a spatial correlation between krill and chlorophyll a distributions. It is therefore considered that both hydrodynamic and food environments may accumulate krill. a regular fishing ground on the northern shelf of Livingston In Island, krill abundance increased 3.4-fold over the 40-day period during late December 1990 - early February 1991. This increased abundance (157 g/m2) in early February 1991 was 54 % lower than the estimate obtained there in late January 1988. Information from krill predators’ studies and krill fishery have also indicated that krill abundance was lower during the first half of 1990/91 summer than expected from other season. Therefore, the estimate of total krill biomass throughout the shelf waters in late January 1991, i.e. 1.78 million t (56 g/m2), should be regarded as the lower level for the shelf waters at this time of the season. Krill abundance is reported to have increased to the normal level from mid-February onward.