The current status of Unio crassus populations in the watercourses of the right-bank Ukraine was studied. Of 191 localities surveyed, the species was found at 30, across four river basins: the Danube, Dniester and Southern Bug basins and the Prypiat sub-basin. Co-occurrence of U. crassus with other Unionidae was analysed using the Jaccard and Sørensen–Dice indices; the highest values were found with U. pictorum (\(K_J\) = 0.59) and U. tumidus (\(K_J\) = 0.52), indicating similar habitat requirements. Comparative assessment of the combined demographic and morphometric indicators distinguished three population groups. The populations of the Ubort (Sushchany) and Stalineshti (Mamalyha) rivers showed the most favourable demographic and morphometric indicators and are regarded as candidate donor populations for species recovery. Morphometric analysis revealed variability in shell size, with the largest shells in the Stalineshti River population (mean shell length \(68.86 \pm 2.52\) mm). A comparison of the Prypiat sub-basin between 2007 and 2011 and 2020–2024 indicated an overall decline in occurrence (from 40.6% to 20.0% of sites; Fisher’s exact test, \(p=0.071\)), including loss of detections in the Noryn River and a reduction in the Ubort River; small, uneven samples and differences in survey effort mean these trends should be interpreted with caution. The results can inform the planning of protected areas, particularly Emerald Network sites, the development of species management plans, and the adaptation of European U. crassus conservation experience to Ukraine.
