Several genetic and nongenetic benefits have been proposed to explain multiple mating (polyandry) in animals, to compensate for costs associated with obtaining additional mates. The most prominent hypotheses stress the benefits of increased genetic diversity. In social insects, queens of most species mate only once or have effective mating frequencies close to one. Yet, in a few species of ants, bees, and wasps, polyandry is the rule. In these species, colonies are usually headed by a single queen, whereas multiple queening adds diversity in several of the remaining species, especially in ants. Here we investigated mating frequency, inbreeding and relatedness between the queens and their mates in the polygynous ant Plagiolepis pygmaea, and the effect of polyandry on the genetic diversity as a function of the effective population size of individual colonies. Our results show that polyandry occurs frequently in the species. However, queens are frequently inseminated by close relatives, and additional sires add little genetic diversity among offspring of individual queens. In addition, the increase in diversity at the colony level is only marginal. Hence, contrary to established notions, polyandry in P. pygmaea seems not to be driven by substantial benefits of genetic diversity. Nonetheless, very small or as yet unidentified genetic benefits to one party (males, workers, queens) in conjunction with low costs of mating may favor polyandry. Alternatively, nongenetic factors, such as convenience polyandry, may be more important than genetic factors in promoting polyandry in P. pygmaea.