High mortality of yesso scallop at the age of 0+–2+ is detected recently on aquaculture farms in Primorye (Japan Sea). Pathological changes and death of the mollusks are observed more and more frequently with increasing of their density and number of plantations. High values of damage and mortality of mollusks grown in cages are not caused by violation of the cultivation technology but by influence of pathogenic species. At the eastern coast of Primorye, high mortality of the scallop spat occurs because of their soft tissue detachment from the shells by alveolates of gen. Perkinsus (for instance, up to 78 % of scallops at the age of 5–6 months are damaged in the Moryak-Rybolov Bay). In Peter the Great Bay, 58–80 % of scallops at the age 2+ grown in cages have blackening, stratification and abnormal shape of shells, besides, all mollusks from the Voevoda Bight aged 2+ have large fouling (on average 8 % of their total weight). In the Nakhodka Bay, located between these areas, 60 % of the mollusks suffer both from soft tissue detachment and abnormal shape of their shells. In total, representatives of 14 groups of parasites are found in tissues and on shells of cultivated yesso scallop, including epibionts (macroalgae, sponges, ascidians, shellfish, bivalves and gastropods, polychaetes, barnacles) and pathogens (fungi, flagellates, ciliates, turbellaria, alveolates, bacteria). Some parasites are found on gills of scallops (ciliates Trichodina sp. for 22.2 % of mollusks, turbellaria for 37.1 %, flagellates for 100 %), whereas zoosporangia Perkinsus infect mantle, gills and muscle for 55.5 % of mollusks. However, the scallops collected from nature habitats in Peter the Great Bay, have no external signs of infection in the first year of life, the scallops at the age 1+ year have no lesions and changes of the shell shape, with minor exclusion, and only at the age 2+ their shells are damaged for 58–80 % of mollusks. The fouling of shell is not a direct cause of the scallops death, but contributes to their weakening and development of diseases. So far as external damages of shells are highly variable, mixed infection of cultivated mollusks can be assumed.