Several newly‐mated females of the European slave‐making antPolyergus rufescensusually participate in slave raids and penetrate together the same target colony. This fact could result in a pleometrotic colony foundation. In the laboratory, two newly‐mated females ofP. rufescenswere introduced simultaneously into queenright artificial colonies ofFormica cunicularia, the slave species present in the natal dulotic nest. The experiments were carried out using artificial colonies of two kinds: ‘simple’ (a nest connected with a foraging arena) and ‘complex’ (a simple colony connected with an empty nest serving as a flight‐chamber). In both cases, the two parasitic females never cooperated in attacking and killing theFormicaqueen. In fact, only one of the two was able to obtain adoption by the resident workers, the other being killed by them. Egg‐laying by the usurping queens was observed the following spring, and in the summer new mixed colonies developed: this is the ultimate criterion for looking upon colony foundation by the parasite as successful. In our experiments, pleometrosis did not occur and the dependent mode of colony foundation typical ofP. rufescensled to a primary monogyny.