Analysis of the age structure of parthenitae of trematodes of sympatric populations of L. obtusata and L. saxatilis was conducted in 1983 for 6 localities in Kandalaksha Bay of the White Sea. 10 species of trematodes were found. The main populational differences in the level of infection are caused by Microphallus piriformes. Species with secondary dispersion in the life cycle are characterized by the increase in the extent of infection in populations with age. In Microphallidae species of "pygmaeus" group the age structure of infection depends on the overage infection of populations: the increase in the parasitic press leads to the maximum turning towards the young part of the snail's population. The establishment of the age structure of infection is considered to be one of the regulation mechanisms in parasite--host relations on the populational level.