Earlier studies did not pay attention to differences in size composition of Euphausia superba populations by sex and its seasonal variability. In this paper variations in size and sex composition of samples from commercial catches are analyzed over the reproductive season, using samples from different years. The observed variations were related to stages of krill maturity during the reproductive cycle. Information on size composition by sex is considered to be vital for an understanding of krill population structure. It is suggested that duties of biologist-observers aboard commercial vessels should include not only measurement but also sex determination of adult krill.