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Аннотация The freshwater invasive mollusc Dreissena polymorpha and its host-specific commensal ciliate Conchophthirus acuminatus represent a convenient model to study various aspects of symbiotic relationships. This study was conducted to test whether the nutrient enrichment of waterbodies inhabited by D. polymorpha is associated with C. acuminatus infection. Three lakes differing in their trophic conditions were simultaneously sampled to estimate the levels of C. acuminatus infection in D. polymorpha (eutrophic Lake Batorino, moderately eutrophic Lake Myastro, and mesotrophic Lake Naroch, Republic of Belarus). Although the percentage of infected molluscs did not differ among these waterbodies, the average number of ciliates per host showed a significant increase with respect to the lake trophic status. This pattern could be explained by favorable feeding conditions experienced by D. polymorpha and its commensal C. acuminatus in waters of higher trophy. In particular, the average size of D. polymorpha (i.e., carrying capacity for the infection) significantly positively correlated with the lake trophic status. Higher trophy could also directly affect reproduction potential of the ciliate C. acuminatus, which supposedly feeds on bacteria, algae, and other organic particles filtered out from the water by its host. The results of this study may apply to a wider spectrum of similar commensal associations in fresh waters.