The number of mollusc farms off the coast of Crimea and the Caucasus has increased significantly in recent years. The cultivation of the Pacific oyster Crassostrea gigas (Thunberg, 1793) requires monitoring of mollusc health and parasitological control of mariculture farms. The aim of this work was to study species composition of epibionts and endobionts, associated with shells of cultivated oyster C. gigas, as well as to identify species, damaging shells. Commercial oysters with visual shell damage were collected on a mariculture farm in the Donuzlav Bay (Crimea, the Black Sea) and brought to the laboratory alive chilled. As a result of 22 oysters’ examination, 14 macrozoobenthos species and live specimens of rock-boring mollusc Petricola lithophaga (Retzius, 1788) were found. The size of rock-borers varied 9 to 16 mm; their age was about two years. Prolonged presence of P. lithophaga inside oyster valves can cause degradation of shell calcareous layer and even death of the mollusc host; this fact is of great importance for the Black Sea mariculture. Considering P. lithophaga annual development cycle, during the period of mass larval settlement (July to October), it is recommended to inspect the shells of cultivated oysters. Further detailed studies will allow to develop measures for prevention and protection of bivalve molluscs from infestation with P. lithophaga.