The humoral immune response of roach (Rutilus rutilus) to cercariae of the digenean trematode, Rhipidocotyle fennica, was studied. Antibodies against R. fennica were found in wild roach in lakes where fish are infected by the parasite. Antibody levels were higher in sera collected in September than in sera collected in June, due to infection of R. fennica during the late summer. In experimental aquarium studies, roach immunized with homogenized cercariae produced antibodies against R. fennica. An especially strong response was elicited by infecting fish with living cercariae emerging from infected clams. The specificity of the antibodies, as shown in Western blots, was different between fish immunized with homogenized cercariae and those fish infected with living cercariae. The specificity and amount of antibodies depended on the route of immunization. The challenge experiment with R. fennica indicated that previous infection of fish gives some protection against R. fennica.