Water quality in the Uday River within the territory of the Pyryatyn National Natural Park was assessed based on results of long-term investigations of macrozoobenthos structural characteristics. It has been found that indicators of eutrophic waters (in terms of their numbers) and a-oligosaprobic organisms prevailed at many stations. The waters located downstream of the town of Pyryatyn proved to be most contaminated. They were assessed as polytrophic and β-mesosaprobic.