Traditionally, many small-sized copepod species are considered to be widespread, bipolar or cosmopoli- tan. However, these large-scale distribution patterns need to be re-examined in view of increasing evi- dence of cryptic and pseudo-cryptic speciation in pelagic copepods. Here, we present a phylogeographic study of Oithona similis s.l. populations from the Arctic Ocean, the Southern Ocean and its northern boundaries, the North Atlantic and the Mediterrranean Sea. O. similis s.l. is considered as one of the most abundant species in temperate to polar oceans and acts as an important link in the trophic network between the microbial loop and higher trophic levels such as fish larvae. Two gene frag- ments were analysed: the mitochondrial cytochrome oxidase c subunit I (COI), and the nuclear ribosomal 28 S genetic marker. Seven distinct, geographically delimitated, mitochondrial lineages could be identi- fied, with divergences among the lineages ranging from 8 to 24%, thus representing most likely cryptic or pseudocryptic species within O. similis s.l. Four lineages were identified within or close to the borders of the Southern Ocean, one lineage in the Arctic Ocean and two lineages in the temperate Northern hemi- sphere. Surprisingly the Arctic lineage was more closely related to lineages from the Southern hemi- sphere than to the other lineages from the Northern hemisphere, suggesting that geographic proximity is a rather poor predictor of how closely related the clades are on a genetic level.