Genetic and morphological variability was studied in the tapeworm Proteocephalus percae (Müller, 1780) (Cestoda: Proteocephalidae), a parasite of perch (Perca fluviatilis L.). Geographic variation in isoenzyme patterns, random amplified polymorphic DNA (RAPD) profiles, and morphological characters were found within six natural populations of P. percae from Slovakia and Switzerland. All approaches used showed that similarity was substantially higher among Swiss populations than among Slovak populations. Proteocephalus percae specimens from the Dobšiná water reservoir (Slovakia) differed significantly from all other population sample sets in that they exclusively expressed the genotype Acp⁹²/Acp⁹² and possessed a specific RAPD profile after amplification with the primer p78 and distinct measurements of the scolex (scolex width and apical sucker). The unique position of the P. percae population from Dobšiná could be related to the contamination of this locality with heavy metals (cadmium), through the possible impact of these metals on the alterations observed. The taxonomic relevance and stability of the morphological characters related to the scolex in the Proteocephalus tapeworm group is confirmed.