Among Unionidae clams, the parasite Paraergasilus rylovi was specific to Anodonta piscinalis. The parasite occurred in southern Finland. The mean intensity of infection of P. rylovi was higher in lake populations compared to rivers. Within a lake, P. rylovi was more abundant at moderate depth compared to shallow or deep habitats. Paraergasilus rylovi produced eggs between June and August and new infections were established in clams in August. The abundance of P. rylovi increased with proportionate burrowing depth of A. piscinalis. There was an inverse relationship between P. rylovi intensity and age/size of the host, but no dependence between the intensity of infection and host sex. Female A. piscinalis, which brooded glochidia larvae were more abundantly infected by P. rylovi than non-reproducing females. The parasites were possible to expel from the gills of A. piscinalis by using high water temperature or low oxygen. Paraergasilus rylovi were expelled before an experiment conducted to study local adaptation of P. rylovi. In the reciprocal transplant experiment between two locations, both infectivity and reproductive performance of P. rylovi were higher in home-clams in both transplant sites, indicating that P. rylovi may be genetically specialized to its local host populations. In other a field experiment, anoxia-stressed clams were more susceptible to infection by P. rylovi than unstressed individuals when exposed to parasites about 11 months after the stress treatment. The stressed clams also showed significantly lower growth, lower reproduction and lower survival than the unstressed clams. The result indicates that stress may have long-lasting effect on host susceptibility to parasitism in natural populations.