EN
The thick-shelled mussel Unio crassus Philipsson is a legally protected, endangered species. This paper reports on its previously unknown localities in Romincka Forest (NE. Poland). Out of the four largest rivers of the Forest, live individuals were found in the Bludzia and Błędzianka; only empty shells were found In the Jarka River, and neither live mussels nor shells were found in the Żytkiejmska Struga. U. crassus avoids canalised river sections, while water quality does not seem to affect its distribution in the examined rivers.