The circulation of the contents of Cu, Cd, and As was traced in the system of Chondrostoma nasus – Pomphorhynchus laevis – Contracaecum sp. – water – sediments from the Danube River in the Kudelin biotope. New data on the content of the elements examined are presented. The concentrations of the three elements in the materials studied, which included tissues, organs, and parasites of common nase, and water and sediments from the Kudelin biotope, were higher than the limits set forth in Bulgarian and international legislative documents. The bioindicator role of the tissues, organs, and parasites examined was revealed, and the liver of common nase and Contracaecum sp. were good bioindicators for Cd content, while P. laevis was a good bioindicator for As content.