Abstract: The present study attempts to analyse the aggressive behaviour of the two European supercolonies of the Argentine ant, Linepithema humile, in confrontations between them and with several native ant species, to assess if there are differences in their aggressiveness, and consequently, in their competitive ability. Workers from the Main supercolony were more aggressive than workers from the Catalan supercolony. They showed a higher significant aggressiveness index in confrontations with workers from the Catalan supercolony and with two of the three native ant species studied: in Tapinoma nigerrimum (Nylander, 1856) and Lasius cinereus Seifert, 1992, but not in Pheidole pallidula (Nylander, 1849). They also mainly initiated the aggressive encounters and responded aggressively to attacks during confrontations both with the Catalan supercolony and with the native ant species. As compared to the Main supercolony, workers from the Catalan one showed a reduced aggressive behaviour in confrontations with the native ant species studied. Moreover, the native ant species attacked workers from the Catalan supercolony more frequently than workers from the Main one. These results may support the hypothesis of a weaker competitive ability of the Catalan supercolony, and therefore, a minor invasiveness power, and could also partially explain why that supercolony is clearly less widely distributed throughout Europe than the Main supercolony.