Based on field observations performed in 1999 and 2000, the exotic amphipod Dikerogammarus villosus is now well established throughout the navigable sector of the River Moselle in France. D. villosus co-occurs with Gammarus pulex, G. tigrinus and D. haemobaphes, another recent exotic species found in this tributary of the River Rhine. The maximal size of individuals of D. villosus collected (15.7 mm) is lower in the River Moselle than in other basins, where this species could reach up to 30 mm (Neseman et al. 1995). Nevertheless it represents the largest gammarids found in the French sector of the River Moselle. Other particular traits are its wide phenotypic plasticity, varying from a melanic to a striped form, and its predatory behavior. These particular traits may favour the establishment of this species on a recipient ecosystem. The occurrence of D. villosus was noted in the Alsace Canal and in the River Rhone, showing that its immigration across French hydrosystems is still in progress.