The life-cycles of two Rhipidocotyle species and their ecology at different life-cycle stages, especially the host-parasite relationships in the first intermediate host, were studied during 1987-89 in Lake Saravesi, central Finland, and Lake Kuivasjärvi, northern Finland. At the beginning of the study, bucephalid digeneans were found in Rutilus rutilus and adult bucephalids of the genus Rhipidocotyle were found in predatory fishes from Lake Saravesi. However, two bucephalid species, the cercariae of the first having long, filamentous furcae (Type A) and the second having short, stout furcae (Type B), were detected in Anodonta anatina from the lake. Experimental life-cycle studies on the parasites had to be established, in which uninfected roach were infested with a known type of cercaria and the roach were fed to uninfected Esox lucius or Perea fluviatilis. Type B was revealed to be R. campanula (Dujardin, 1845). Type A also belonged to the genus Rhipidocotyle, being a species new to science and was described under name R. fennica Gibson, Taskinen & Valtonen, 1992. Both species used A. anatina as the first intermediate host and R. rutilus as the second intermediate host. The final hosts of R. fennica and R. campanula were E. lucius and P. fluviatilis, respectively. The metacercariae of R. fennica, which mainly occurred encysted in the skin and fins of R. rutilus, could be distinguished by the more psterior position of the pharynx along the body when compared to R. campanula which, on the other hand, mainly occurred encysted in the gills of R. rutilus. Adult R. fennica could be separated from R. campanula e.g. by its smaller egg size and more posterior position of the anterior margin of the vitelline fields. The prevalences of R. fennica and R. campanula in A. anatina from Lake Saravesi (n=l,157) were 33.2 % and 1.0 %, respectively. Pseudanodonta complanata (n=29), Unio tumidus and U. pictorum (n=65) from Lake Saravesi were not infected. The prevalence of R. campanula in A. anatina from Lake Kuivasjärvi (n=l,486) was 4.7 % and R. fennica did not occur in that lake. A. anatina collected from Lake Saravesi ranged from 1 to 15 years and from 21 to 112 mm. The clam specimens ≤ 3 years or ≤ 51 mm were not infected with R. fennica. The prevalence of R. fennica in A. anatina from Lake Saravesi was higher in the littoral zone, among older, bigger, fast growing and female clams.