Two intraspecific groups of the plain sculpin Myoxocephalus jaok inhabiting the Russian territoral waters in the Sea of Japan, representing the northern and southern species groups, have been revealed. In a summer season, the species groups are separated from each other by a wide water area extending from 45° to 48° N. The northern species-group nucleus in this period is located in the Tatar Strait inlet, while the southern species group inhabits the Peter the Great Bay. In the summertime, the plain sculpin prefers the upper slope shelf area with water column depths ranged from 80 m (for the southern species group) to 60 m (for the northern species group). The juvenile fish tend to be found in shallower waters as opposed to the mature fish. In a cold season, the majority of fish stocks of the southern species group is still congregating in the Peter the Great Bay, shifting to the shelf edge and the continental slope. Some porton of male fish remains at spawning grounds in the coastal zone to the springtime to guard egg clutches. The specimens of the nothern group leave the shallow-water Tatar Bay inlet at massive level to congregate over the winter in the locations of a deep-water area off the mid-coastal zone of the Sakhalin Island. The specimens of the northern group reach the body length of 67 cm, while the sizes in the southern fish group are larger, reaching 75 cm in length.