Enzyme electrophoresis has been used to examine genetic diversity in a population of Proteocephalus exiguus La Rue, 1911 (Cestoda: Proteocephalidae), parasitizing salmonid and coregonid fishes. Among 16 loci tested, three polymorphic loci (Ada, Got, Pgm-2) were found. Six different genotypes at the Got locus distributed in Hardy-Weinberg equilibrium suggest remarkable genetic flexibility of P. exiguus. Balancing selection is proposed as the mechanism maintaining genetic variation within the species. Data of genetic variability parameters (Ho = 0.064; He = 0.07; P = 0.19) and outcrossing rate (t = 0.842) of P. exiguus population have been provided. Possible factors affecting these data are discussed.