Abstract: Colony-level sex ratios of Anergates atratulus (Schenck, 1852) were found to be significantly more female-biased than expected from the socially parasitic life history of this inquiline ant. This might reflect either the sex ratio preferences of its Tetramorium host workers or local mate competition. Histological analyses of A. atratulus males showed that spermatogenesis is completed by the time they reach sexual maturity. Nevertheless, because their gasters are almost completely filled by the seminal vesicles with large masses of mature and immature sperm cells, it appears that they repeatedly replenish the sperm supply in their seminal vesicles after mating. Though we could observe a maximum of only three successful copulations per male in a small laboratory colony, males are therefore presumably much more potent than most other ant males.