An implicit assumption of sex allocation theories in ants is that workers discriminate the sex of brood to manipulate the juvenile forms towards the adequate investment ratio. This study deals with this possibility by testing the capability of workers of the Argentine ant Linepithema humile to recognize the sex of larvae. Our data demonstrate for the first time larval recognition followed by male brood elimination. Batches of eggs and larvae of unknown sex were removed at different stages from queenright societies in which males pupae never appear. This brood was transferred in queenless colony fragments and we checked the number of male and female pupae obtained, and thus the proportion of male brood still alive in queenright colonies at the time of transfer. The results showed that haploid eggs were not eliminated whereas about half of the male brood was destroyed just after hatching. The remaining male brood was cannibalized later before pupation, but male pupae were spared. Hence, the combined data indicate that workers are able to identify the sex of the brood and to selectively eliminate male larvae. This behaviour appears to fit with the values of the secondary sex ratio observed in this species. Early sex brood discrimination and elimination of male larvae are discussed in connection with ergonomic costs as well as with the life history of the Argentine ant.