All ants are social, except for specific socially parasitic ants, inquilines. We will focus on inquilinism, the ultimate form of social parasitism, where the inquiline species practically lack own workforce and depend on their host for all colony tasks throughout their life cycle. We will concentrate on selected inquilines of the genus Myrmica. In this genus, several inquiline–host pairs are phylogenetically closest relatives to each other, such pairs having originated independently. This strongly suggests sympatric speciation of the inquiline from its host. We will present ecological, chemical and genetic support for this hypothesis. Finally, we will discuss the debated status of the inquiline of Myrmica rubra—whether it is an intraspecific parasite, an evolutionary lineage speciating from its host, or a good, though young, species.