peer reviewed ; Populations of Unio crassus (Bivalvia: Unionidae) are sharply declining in Europe. To reverse this trend, many conservation actions are being implemented. However, the effectiveness of these actions depends on our knowledge of the ecology of the species. This study focuses on the hydromorphological preferences of U. crassus in a river in south-eastern Belgium (the Sûre River). Depth and flow velocities were mapped at 3 study sites. These data were then cross-referenced with the spatial distribution of individuals at these sites in order to relate the use and availability of microhabitats. In addition, a fine sediment sampling system was developed to quantify the clogging of the substrate in which the species lives. The individuals are mostly located in areas with slow currents (< 0.4 m/s) with intermediate depths (0.4 to 0.8 m) for flows below the mean annual flow. Depth appears to be the main parameter governing the spatial distribution of individuals at the local scale compared to flow velocity and proximity to the bank. The substrates occupied by U. crassus are varied, with a higher density of individuals in deposits of about 3 mm of D50 and 26 mm of D99. The infiltration of fine sediments into the gravel-bed layer is relatively low at the investigated sites although variability was observed between the study periods. This study confirms the ability of U. crassus to thrive under a variety of hydromorphological conditions, while emphasizing the importance of lentic and relatively stable microhabitats. ; Actuellement en Europe, les populations d’Unio crassus (Bivalvia : Unionidae) déclinent rapidement. Afin de contrer cette tendance, de nombreuses actions de conservation sont mises en place. Toutefois, l’efficacité de ces actions est dépendante de notre connaissance de l’écologie de l’espèce. Cette étude se concentre sur les préférences hydromorphologiques d’U. crassus dans une rivière du sud-est de la Belgique (la Sûre). La profondeur et les vitesses de courant ont été cartographiées au ...