In eusocial insects, social parasitism - the exploitation of the host’s brood care behaviour for survival and reproduction – can occur either within or between species. Parasite queens invade host nests and aggressively replace the resident queen. While the adoption of conspecific queens is a common feature of species with multi-queen colonies (polygyny), the origin of parasitic founding is not fully understood. Functionally monogynous ants, in which nestmate queens establish social and reproductive hierarchies by biting and antennal boxing, might provide a link between peaceful adoption and social parasitism. In this study we aimed to evaluate whether alien queens might usurp colonies of the functionally monogynous ant Leptothorax gredleri. Ovary dissection of queens from 33 nests showed that multiple queens with developed ovaries can occasionally cooccur in the same nest. Genetic analysis highlighted frequent exchanges of the dominant queens. We evidenced that alien queens may take over reproduction, suggesting possible occurrences of intraspecific social parasitism in L. gredleri.