The size-differentiated sex ratio (proportion of males: POM) of Antarctic krill was examined with an extensive dataset derived from scientific surveys in the Indian Ocean sector and the southwest Atlantic sector, and from the krill fishery in the Southern Ocean. The percentage of males in size classes of adult krill was generally high in krill of 30-35 mm total length, always low in 38-42 mm krill, sometimes showed higher values in 45-50 mm krill, but always decreased in the largest krill (>50 mm). This pattern was reproduced by a model simulation that assumed faster growth and a shorter life span for males when compared to females. These results suggest that the numbers of males should decline with time unless new recruits enter the population. Indeed, inter-annual variations in the proportion of males from the field (net collected data and penguin diet data) showed a decline in proportion of males when several years of low recruitment followed a recruitment pulse. These results lead us to conclude that male krill grow faster and have a shorter lifespan than females in the natural environment.