Abstract <p><i>Calyptogena pacifica</i> (Bivalvia: Vesicomyidae: Pliocardiinae) is an amphi-Pacific bathyal chemosymbiotic species living near cold methane seeps, hydrothermal vents, and whale falls. In the present study, the genetic diversity of <i>C. pacifica</i> on the scale of the entire geographic range of the species is addressed for the first time using a partial sequence of the cytochrome <i>c</i> oxidase subunit I (<i>COI</i>) mitochondrial gene. The material includes both our own data obtained during a study of bivalve mollusks from the Koryak slope and the Piip Volcano, Bering Sea, and information on <i>COI</i> sequences of bivalves from five East Pacific regions published in GenBank. A total of 12 haplotypes have been identified, of which 2 are from both the West Pacific (Bering Sea) and the East Pacific, 1 is only from the West Pacific, and 9 are only from the East Pacific. The results of analysis of molecular variance have not revealed significant genetic differences both between the samples from the Koryak slope and the Piip Volcano and between the combined samples from the West and East Pacific. Both groups of haplotypes identified by the phylogenetic analysis are not associated with any geographic area. These data suggest no significant genetic differentiation of <i>C. pacifica</i> populations from the Bering Sea and the East Pacific in <i>COI</i> despite the spatial isolation of the habitats that are suitable for this bivalve species.</p>