1) The impact of a dominant ant species, like Formica fiolyctena, on the ant fauna as a whole depends on the density and the distribution of the workers. Alien ant societies, like Myrmica, which are attacked by wood ants, can only survive in areas where the density of the wood ants is very low. 2) In the order: Formica polyctenalf. rufa, F. fusca, Lasius fuliginosus, L. niger, Myrmica, Tetramorium and Leptothorax there is a progressive decrease in the overlap of food requirements with F fiolyctena and also in the aggressiveness of F. polyctena against them. However, this should not be used as circumstantial evidence to support the hypothesis that strong competitors are more aggressive to each other than to weak competitors, because the aggressive activities of all these species may not be grouped under a single heading. To test the hypothesis we should compare intraspecific ter- ritorial aggression with interspecific territorial aggression. 3) The behavioral patterns of aggressive encounters between wood ants and other ant species are often the same as those exhibited by conspecific rivals, but the cause and function may differ. 4) A distinction has been made between aggression that is directly responsible for increasing the supply of prey to the society, i. 'predator aggression', and aggression directed to the defence of the foraging area, i.e. `territorial aggression'. 5) In early spring fights between wood ants can escalate into a war resulting in a loss of thousands of workers daily. Sometimes a neighbouring wood ant nest or a nest of another species is robbed during this time, i. e. in a period when the demand for prey is high. It is concluded that warfare and nest robbing by Formica polyctena can be con- sidered as a form of predation and the aggression can be regarded as predatory aggres- sion. 6) Predatory aggression by Formica polyctena is restricted to periods, such as the spring, when the demand for prey is high, whereas territorial conflicts, both within and between species, can occur at any time during their active period.