The material was sampled in the dam reservoir ,,Przeczyce" (the Silesian Upland, rnesoregion of the Garb Tarnogórski) and in 9 fish-culture ponds in the Gatine Region (Department Deux-Sevres) in west-central France. The dam reservoir in Przeczyce is a drinking water reservoir for inhabitants of the northern part of the Upper-Silesian Industrial Region. The surveyed ponds were utilized for extensive fish culture. A total of 218 tench, at the age of 1-6 years, were sampled from the dam reservoir, while 247 fish of the same species, representing the same age groups, were caught from the fish ponds studied. Prevalence of Ergasilus sieboldi in examined tench populations was respectively 42.1 % in the dam reservoir, and 42.4% in the ponds. No significant differences in the body weight and standard length were found between infected and non-infected fish representing the dam reservoir and the ponds with complete array of fish species. In those ponds where predatory fish were eliminated, as a result of the interventional fishing, infected tench were significantly (p < 0.01) lighter and shorter in comparison to non-infected individuals. A significant (p < 0.01), although relatively weak, positive con-elation was observed between infection intensity of Ergasilus sieboldi and the body weight and the standard length of the fish infected (r = 0.5956 and r = 0.7398, respectively). No significant relationship was found between intensity of the infection and the condition factor (K= 100*W*L'', where: W - body weight, L- standard length) in fish examined.