The results of a 14-year survey of the populations of L. saxatilis and L. obtusata from one locality in the Kandalaksha Bay of the White Sea are presented. The periwinkles were found to be hosts of 10 castrating trematode species. Microphallus piriformes was the most abundant species with infection prevalence often exceeding 50%. Year-to-year variation of the infection prevalence was correlated in the two host species, probably reflecting fluctuations in the influx of trematode eggs to the shore. Long-term changes in population density are associated with the variation of the age structure of the winkles, suggesting a dependence of the population density on the success of the recruitment. Fluctuations in abundance and age composition were correlated in L. saxatilis sub-populations inhabiting macrophytes, gravel and stone surfaces, but were asynchronous with changes in abundance and age composition of L. obtusata population. No significant influence of variations in trematode infection upon the hosts' abundance or age structure was found in either species of the winkles. It is assumed that despite a strong negative influence of trematodes on reproduction of the host individual, there are no disastrous consequences for the population level; this, in turn, suggests the existence of some compensating mechanisms ensuring a stability of the host-parasite co-action.