Anisakis simplex s.s. specimens from Gadus morhua, Gadus ogac, Reinhardtius hippoglossoides, Clupea harengus, Micromesistius poutassou, Pleurogrammus monopterygius, and Oncorhynchus nerka, caught in the north Atlantic and Pacific, were examined. Identity of the parasite species was confirmed using a molecular key. All specimens were used for amplifying a part of the non-coding sequence of the AT-rich region, flanked by the nad6-nad4L-trnW-trnE-rrnS-trnS2, and trnN-trnY-nad1 genes. This part of the mtDNA was chosen as a promising fragment, which could yield information regarding population differentiation. The number of samples and their geographical origin was sufficient to compare the 2 regions. The Bayesian analysis provided strong support for a population split into 2 groups, one representing A. simplex s.s. samples from the Pacific, and the other representing all the Atlantic samples. It is highly probable that observed subpopulations were constituted after the connection between the Atlantic and Pacific had been closed and movement between these 2 oceans was prevented.