High abundance of the radiolarian Cycladophora davisiana in the Sea of Okhotsk has been observed at the Okhotsk Sea Intermediate Water (OSIW). This water is formed by brine rejection in the polynya and is characterized by cold, oxygenated conditions, with high a microbial content. The C. davisiana abundance recorded in three sediment cores from the southwestern part of the sea implies ventilation changes of the OSIW during the last 7.5 kyrs. The ventilation had significantly increased during the interval from 7.5 to 3.5 ka corresponding to a warm period, but declined during 3 to 2 ka and at 0.3--0.4 ka coincident with cold periods such as the Neoglaciation and Little Ice Age, respectively. The declined ventilation might be caused by reduction of the polynyas with the wind vector change from a northerly to an easterly direction relating to the southward shift of the Aleutian Low during the strong winter.