Population biology of Antarctic krill, Euphausia superba, was studied from samples taken in 1991 during krill fishing operations around the South Shetland Islands on board the Chilean factory vessel Kirishima. Hauls were made using a commercial midwater trawl (mouth area approximately 40 x 40 m and mesh size from 1.5 to 3 cm). Two subsamples of 100 specimens each were taken from 50 samples and analysed. The fishing ground was divided into two areas: Area A, north of the South Shetland Islands; and Area B, north of Elephant Island. The samples were grouped by time of capture: daytime, twilight and night-time. The specimens were measured (total length, TL) to the nearest millimetre and weighed (wet weight) to the nearest 0.01 g. Mean catch-per-hour and mean catch-per-towing time were determined from a total of 419 hauls. In Area A, a unimodal size frequency distribution was found; the size range was between 30 and 55 mm TL, with a mean TL of 45 mm for females and 48 mm for males. A very weak mode for juvenile specimens between 26 and 36 mm TL was also found. The sex composition was 65.1% females, 34.4% males and 1.4% juveniles. Of the females sampled, 25.2% bore spermatophores. Although the smallest specimen found with a spermatophore had a 36.5 mm TL, 80% of the females with spermatophores had a TL larger than 45 mm. In Area B, a bimodal size frequency distribution and a larger size range were found, with one mode between 32 and 55 mm TL (mean length 43 mm for females and 46 mm for males), and the other (modal length 32 mm) for juvenile specimens between 20 and 39 mm TL. Females comprised 47.1%, males 40% and juveniles 12.9%. Of the females sampled, 27.1% bore spermatophores, with a size range between 35.4 and 56 mm TL, although 80% of them had a TL larger than 45 mm. The size frequency distribution showed no significant differences between the three time periods. However, when the sex composition is considered, males are more abundant in night-time catches while females are more abundant during daytime catches, thus showing a different trend. Considering all catches, the yield in terms of tonnes-per-mile and tonnes-per-hour was higher during the daytime than during twilight and night in both fishing areas. These daytime catches were also made at consistently greater depths. Research was financed by INACH (Chilean Antarctic Institute).