Sympatric speciation has been suggested for several pairs of social parasites and their hosts. Little is known, however, about how they have become reproductively isolated from each other. The ant Myrmica rubra has an intraspecific microgynous social parasite, which is probably speciating from its macrogynous host morph. Mating of the host and parasite is suggested to be spatially isolated, microgynes mating in the nest and macrogynes usually joining mating swarms. The mating biology of the microgyne is, however, still poorly known. We studied mating isolation and potential gene flow between these morphs by assessing whether host males are produced in naturally parasitized nests, and by testing experimentally cross-mating of the morphs when brought together. Both parasite and host males mated with both female morphs, but host males seemed to mate more often than the parasite males with their own kind. Although most males in the parasitized nests were produced by the parasites, the host queens and workers also produced some males, thus providing the morphs an opportunity for cross-mating. Previous studies have shown, however, that genetic divergence between sympatric parasite and host populations is substantial, which indicates that hybridization is rare. This implies that even if cross-mating between the morphs took place in the wild, introgression would be weak. Our results, together with earlier studies, indicate that the parasite and host of M. rubra are not fully reproductively isolated, even though gene flow between the morphs is restricted enough to allow ongoing speciation of the parasite.