The occurrence of natural mixed colonies of Leptothorax ambiguus, L. curvispinosus and L. longispinosus is described; and the results of laboratory experiments in which such mixed colonies were produced as a consequence of territorial fighting are presented. Territorial fighting is probably an important preadaptation for the evolution of slavery in the Leptothoracini and other ant groups. The possible roles of polygyny and polydomy in the evolution of slavery are also discussed.