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The euphausiid genus Stylocheiron includes species with biogeographical distributions spanning multiple ocean basins. Despite their circumglobal distributions, the species show low levels of genetic diversity and little or no evidence of population structure based on the mitochondrial cytochrome oxidase I (COI) barcode region, with the exception of a possible cryptic species within Stylocheiron affine. Stylocheiron elongatum showed < 1% variation of the COI barcode region among populations in different ocean basins, but analysis of samples collected from the Florida Current (February, 1993) and Gulf Stream Meander Region (April, 1993) in the Northwest Atlantic Ocean revealed small-but-significant genetic differentiation between samples based on a different section of COI and mitochondrial cytochrome b (CYB). Both COI and CYB showed large haplotype and small nucleotide diversities, departures from neutral expectations, and haplotype networks consistent with persistent genetic structuring of the species population. These patterns of diversity indicate the presence of selection driving population divergence. We hypothesize that position-keeping by this deep-living, non-migrating euphausiid species may prevent genetic homogenization (panmixia) in the dynamic Gulf Stream System. This study demonstrates the importance of analyzing patterns of genetic diversity and structure at regional and global scales to understand the ecological and evolutionary processes impacting marine zooplankton.