The Antarctic krill, Euphausia superba, is an abundant and key species found in the Southern Ocean that forms dense, discrete swarms. Despite over three decades of research on Antarctic krill, the genetics of individual swarms is yet to be specifically investigated. In this study, we address the genetic diversity, population structure and demographic history of nine Antarctic krill swarms by sequencing 1173 bases of the gene cytochrome c oxidase subunit I (cox1, COI) from 504 individuals. Both haplotype diversity (h=0.9974–1.0000) and nucleotide diversity (À=0.010275–0.011537) of Antarctic krill swarm samples was consistently high compared with populations of other species reported in the literature. Analysis of molecular variance did not show any significant genetic structure, thus implying that the sampled swarms do not appear to reflect discrete genetic units. Fu's Fs and Bayesian Skyride analyses provided strong evidence for a large increase in the population size of Antarctic krill, or selection favouring a particular mitochondrial lineage, within the last few 100 000 years (Pleistocene). The swarm-level results presented in this study not only further our understanding of Antarctic krill biology but, because of the economical importance of this species, also provide data to consider for future krill stock management.