Seasonal variation in the diurnal vertical migration and aggregation of Antarctic krill (Euphausia superba) associated with the daylight phase over the diurnal cycle (i.e. brightness categories) was examined for the Scotia Sea (South Shetland Islands (SS), South Orkney Islands (SO) and South Georgia (SG) areas) using Japanese fishery data. Average trawling depth usually showed a marked diurnal change during summer and winter in SS and SO. Daily average trawling depth tended to be shallower during austral summer and early autumn, deepened gradually from autumn and reached maximum depth in winter in the Scotia Sea, shifting to shallower depths again in early spring. The average catch-per-unit-effort (CPUE) was highest during the day and lowest at night in autumn and winter in each area, but lower during the day in summer in SS and SO. It is suggested that the seasonal and diurnal changes in vertical migration and aggregation of krill are closely related to its feeding and spawning ecology and avoidance of visual predators.