Abstract: In the monogynous ant species Cardiocondyla nigra Forel, 1905, wingless males mate with young female sexuals inside the nest. As a single mother queen produces all sexual offspring of a colony, mating is usually among siblings and a high level of inbreeding is expected. Despite of this, occasional outbreeding has been reported recently for two other monogynous species of the same monophyletic group and with the same mating system ("palearctic clade", Oettler & al. 2010, C. elegans Emery, 1869: Lenoir & al. 2007, C. batesii Forel, 1894: Schrempf & al. 2005). Matings between unrelated individuals appear to be promoted by the active transfer of virgin queens into alien nests in one of these species (C. elegans: Lenoir & al. 2007). In the present study, I investigated the colony and population structure of C. nigra in order to examine whether this phenomenon might also contribute to outbreeding in C. nigra. Data revealed that more than 90% of the queens are polyandrous, and that approximately 34% of the matings are not among siblings. Moreover, in one population, "foreign" female sexuals co-occurred with workers together within one nest chamber. Thus, the exchange of sexual offspring between ant colonies might sustain genetic diversity in monogynous Cardiocondyla species.