Studies were conducted of the content of ferments with which metacercariae of trematodes of the genus Diplostomum affect the tissues of the host's crystalline lens. The following ferments were found in exudates of metacercariae: pepsin, cathepsin, hyaluronidase and lipase. Their activity depends to a great extent on temperature. Considerable changes in the biochemical content of crystalline lens infected with the parasites were found. The character and force of pathogenic effect of diplostomatids on fishes depend on the factors associated with activation and inhibition of ferments of metacercariae, temperature and the presence of ions-inhibitors in particular.