The long-term effect of experimental infection with metacercariae of Himasthla elongata (Mehlis, 1831) (Himasthlidae) on some biochemical indices of its second intermediate host, Mytilus edulis (the content of reduced glutathione, structural and storage lipids, fatty acids of total lipids, and the activity of glutathione-S-transferase, catalase and peroxidase) was investigated. It was shown that the parasitic invasion affects catalase activity in mussel gills and foot, as well as alters the content of some phospholipids and fatty acids, mainly in mussel gills, digestive gland and foot. The decreased catalase activity and elevated levels of lysophosphatidylcholine and sphingomyelin, as well as the low content of polyunsaturated fatty acids (primarily docosahexaenoic acid, 22:6n-3) in the foot of infected mussels generally reflect the presence of H. elongata metacercariae in this organ. At the same time, changes in several biochemical indices in the gills of infected mussels (essentially higher catalase activity, reduced content of cholesterol esters and modifications in the fatty acid composition) indicate a stress effect caused by H. elongata metacercariae. It is assumed that the studied biochemical indices are significantly affected immediately, during penetration of the parasites in the host tissues (within the first day), while the long-term effect of the parasite infection on the second intermediate host (during a year) eventually leads to a stabilization of the host-parasite relationship, including the investigated components of the antioxidant system, as well as the lipid composition. This assumption requires additional research.