The pelagic snail Limacina helicina Phipps, 1774 is widely distributed in high-latitude seas and is a sensitive bioindicator of ocean acidification. It is known that the response patterns to ocean acidification differ among populations within or among species. Thus, it is important to understand their genetic population structure and identify the cryptic species. L. helicina displays different shell morphotypes, called "formae," in different regions (North Atlantic, L. helicina forma helicina Phipps, 1774; Northwest Pacific, L. helicina f. acuta Spoel, 1967; Northeast Pacific, L. helicina f. pacifica Dall, 1871; Okhotsk Sea, L. helicina f. ochotensis Shkoldina, 1999: Spoel, 1967, pp. 257, 349). A recent study by Shimizu et al. (Journal of Molluscan Studies 84:30-37, 2018) showed clear genetic differentiation between two formae, L. helicina forma helicina (Svalbard in the Arctic Ocean) and L. helicina f. acuta (station K2 in the western North Pacific) (phi CT = 0.59282), using partial mitochondrial cytochrome c oxidase subunit I (COI) gene sequences. However, whether other formae of this species are genetically distinct remain unknown. Here we focused on L. helicina f. ochotensis, which inhabits the Sea of Okhotsk, and compared its shell morphology and partial mitochondrial COI gene sequences with another forma, L. helicina f. acuta, which is found in the western North Pacific. We found differences in the shell morphology of L. helicina f. ochotensis collected from the Sea of Okhotsk (off the coast of Monbetsu, Hokkaido, Japan) and L. helicina f. acuta collected from the western North Pacific (station K2). However, molecular analysis of an mtDNA COI region (503 bp) revealed that most individuals collected from Monbetsu (Sea of Okhotsk) were haplotype 1 (75.5%), which is also the major haplotype found in L. helicina f. acuta (western North Pacific). Our results suggest that individuals of L. helicina are frequently dispersed between the Sea of Okhotsk and the western North Pacific by ocean currents. Thus, the two formae, L. helicina f. acuta and L. helicina f. ochotensis, are more likely to be attributable to phenotypic plasticity in response to different environmental parameters, such as temperature and salinity, than to differences in their genetic backgrounds.