Reproductive conflicts are ubiquitous in animals and occur in many different forms. When several reproducing queens co-exist in a same social insect nest, conflicts can evolve between them not only about who is permitted to reproduce in the first place, but also about which kind of offspring is produced. This means that some queens may avoid producing workers and thus not contribute to the maintenance of the nest. Instead they may take advantage of the already established worker force of other queens and focus only (or mainly) on producing sexual offspring. In the ant Myrmica ruginodis, queen size dimorphism has been suggested to be connected to an incipient stage of such parasitic relationship, where the smaller queen morph (microgynes) is assumed to be a parasite of the larger queen morph (macrogynes). Furthermore it has been suggested that such intraspecific parasitism can lead to reproductive isolation between the two size morphs and that it might eventually lead to sympatric speciation between the parasite and the host. In the ant Myrmica ruginodis nests usually belong to one of the morphs, but when the morphs coexist in the same nests, reproductive specialization is a possible mechanism causing genetic divergence between the morphs. To examine possible reproductive specialization of the morphs, we collected queens and their sexual and worker offspring for genetic analysis, focussing especially on nests where both morphs co-existed. We shall determine whether microgyne queens specialize in sexual production in mixed nests and if this phenomenon enhances reproductive isolation between the two size morphs of the ant Myrmica ruginodis.