The myrmicine ant Eutetramorium mocquerysi Emery from Madagascar exhibits a unique social organization. All female individuals are similar in size and appearance; female reproductives with a distinct external morphology do not exist. Based on ovarian anatomy, however, two major types of females can be distinguished: females with six ovarioles and a spermatheca, which can mate and produce diploid offspring, and females with only two ovarioles, which lack a spermatheca but can lay unfertilized eggs. Individuals with three to five ovarioles are rare. Anatomical differences are not correlated with different roles. Both types of females were observed foraging, tending brood, and laying eggs. However, only females with six ovarioles and a spermatheca were the reproductively and socially most dominant individuals. Nestmate antagonism, which for the first time is demonstrated for an ant species belonging neither to the Ponerinae nor the Formicoxenini, consists of biting, antennation bouts, and ritualized dominance postures. In two colonies, removal of the dominant individual resulted in the destruction of all larvae and pupae.