A field experiment was conducted to test the hypothesis that the introduced wetland plant purple loosestrife (Lythrum salicaria L.) was less affected by predators and competition from neighbours than a morphologically similar (i.e. similar seed size, plant height, germination time) native wetland species, blue vervain (Verbena hastata L.). Predator exclusion and neighbour removal treatments were used to assess the effects of predators and competition from Phalaris neighbours on the shoot mass of the two target species, Lythrum and Verbena. In most cases, Lythrum and Verbena were equally affected by potential effects of predation and competition as well as the combined effect of competition and predation, except at one site, where potential competition effects and the combined effect of competition and predation reduced Lythrum's shoot mass more than Verbena's. None of the results of this study indicated that introduced Lythrum was less affected by predators or more competitive than native Verbena.