The technique of nest exchange was used both in the field and in the laboratory onPolistes gallicusto study the behaviour of foundresses in alien nests and the possible changes in the reproductive apparatuses of the workers emerging in the presence of foundresses other than their mother. In the field, nests built on shrubs were removed complete with their plant stems and fixed to plants where other nests had been removed. In the laboratory, foundresses were subjected to a cage exchange. Observation on foundresses at the moment of their first landing on the alien nests showed that these perform a particular behaviour, stroking the abdominal sternites on the surface of the nests and inside the cells with a probable release of secretions. Checks of the immature brood revealed a massive destruction of eggs in the alien nests. Dissection of the workers emerging in the nests in the presence of the alien foundresses indicated that the reproductive apparatus of these females was fairly developed and that most of them actually had eggs in their ovaries that were ready to be laid. These findings raise questions on the possible presence of regulatory pheromones of the queen in the colonies ofPolistesand on the evolution of interspecific social parasitism in these wasps.