Invasive aquatic plants constitute a great problem in many freshwater systems. Although many studies have addressed the potential threats of invasion by exotic species in aquatic ecosystems, few studies have considered the interactions (competition, facilitation, coexistence) between exotic aquatic plants of similar growth form in invaded communities. Our aim was to investigate experimentally the effect of density in monocultures and the effect of neighbour identity and plant density on a focal species in mixed cultures. We used Elodea canadensis, Egeria densa and Lagarosiphon major as model species and conducted two experiments, one in a monoculture and another in a mixed culture following an additive design. Individuals were planted in pots with several treatments based on the identity and density of neighbour species. Our results demonstrated that the development of an invasive submerged plant could be influenced either positively (facilitation) or negatively (competition or inhibition) by the presence of more than one neighbour species of different densities. An increase in density significantly stimulated competition intensity, although the effect of density on the performance of the invader also depended on neighbour identity. A facilitative effect of E. canadensis on the apical growth of E. densa was established. However, despite these results, the competitive outcome also depended on other environmental factors (e.g. underwater radiation), not on plant density and neighbour identity alone.