Abstract: Klára Kiss & Ottília KÓBORI Klára Kiss (contact author), Department of Horticulture, Sapientia University, Sighişoarei 1 / C, 547367, Tirgu Mureş / Corunca, Romania; Department of Ecology, University of Szeged, H-6701, Szeged, P.O. Box 51, Hungary. E-mail: kiss.klara@gmail.com Ottília Kóbori, Department of Taxonomy and Ecology, Babeş Bolyai University, str. Clinicilor 5 - 7, 400006 Cluj Napoca, Romania. Myrmecol. News 10: 111 We studied two populations of Formica pratensis Retzius, 1783 near Cluj Napoca (Romania, Cluj County): in Fânaţele Clujului and in the Hoia Forest. F. pratensis frequently forms polycalic colonies, or supercolonies (Beye & al. 1998, Pirk & al. 2001). Our goal was to determine the social structure of the studied nest-complexes. We predicted that there is a correlation between the intercolonial level of aggression and the spatial distance. We used aggressiveness tests to detect the level of aggression between the nests. Altogether 21 colonies were studied and 103 tests were…