Ergasilus sieboldi infestation of the pikeperch (Stizostedion lucioperca) is extremely common in Lake Balaton. In the summer and autumn, these parasitic copepods have high prevalence and intensity on pikeperch of more than 20 cm body length. Typically, Ergasilus establish themselves in the folds on the external surface of the operculum and on the base of the pectoral fins, and only a small proportion cling to the gill filaments. Infestation is rare and of low intensity in pikeperch of less than 20 cm body length. The Volga pikeperch (Stizostedion volgense) has low-intensity infestation irrespective of age. The lesions caused by copepods present on the operculum are restricted to the epithelium even if infestation is intensive.