Hemocyte phagocytosis assay in vitro revealed no significant differences in the average values of phagocytic activity (PA) and phagocytic index (PI) in samples of the molluscs Modiolus kurilensis that inhabit non impacted water areas (Kievka, Vostok, and Troitsa bays, the Sea of Japan). The only exception was an insignificantly lower value of PI for individuals from Troitsa Bay. The average values of PA for hemocytes of the total population of bivalves from non impacted water areas were 52.0 ± 1.0%. On the basis of the inter annual analysis of PA, the bivalve population from Vostok Bay was chosen as an appropriate reference group that is characterized by relatively stable PA values. In comparison with molluscs from the reference group and other investigated samples, M. kurilensis from the impacted water area (Sportivnaya Gavan, Amursky Bay) had considerably lower values of average PA (36.0 ± 2.0%) and PI (2.1 ± 0.05). The presented cell mediated immunity assay can serve as an effective method for estimation of the physiological state of the bivalve in nat ural and aquacultural populations in order to differentiate molluscs from impacted and non impacted water areas.