Parasitological studies of fish from Lake Kortowskie were carried out from 2001 to 2004. In the current study, 381 fish representing ten species: perch, Perca fluviatilis L., bleak, Alburnus alburnus (L.), ruffe, Gymnocephalus cernuus (L.), silver bream, Abramis bjoercna (L.), pike, Esox lucius L., common gudgeon, Gobio gobio (L.), tench, Tinca tinca (L.), rudd, Scardinius erythrophthalmus (L.), bream, Abramis brama (L.) and roach, Rutilus rutilus (L.) were examined. The aim of the study was to review the parasites of selected fish species and to compare the infestation of bream, perch, and rudd in 1984-88, 1994 and 2001-2003. A total of 51 species of parasites belonging to Monogenea (27), Digenea (6), Cestoda (7), Nematoda (4), Acanthocephala (3), Crustacea (3), and Molusca (1) were recorded. The largest numbers of parasite species were found in roach (18), followed by bleak (14), perch and silver bream (13), bream (11), ruffe and pike (10), and the fewest in common gudgeon (4) and tench and rudd (2). Tetraonchus monenteron, Triaenophorus nodulosus, Ergasilus sieboldi, and molluscs glochidia occurred in 83.3% of the pike. Comparing the bream, perch, and rudd infestation parameters of over twenty years with the current results, it can be concluded that the changes are linked to environmental changes that have occurred as a consequence of the Lake Kortowskie experiment.