Parental care is costly, hence parents may benefit if their offspring are cared for by others. This potential for reproductive parasitism selects for counterstrategies to avoid being exploited, for instance by abandonment of parasitized broods. In the shell-brooding cichlid fish Lamprologus callipterus, nest males can expel females from shells if their broods contain foreign offspring resulting from alternative male mating tactics, which causes the demise of the young¹. Here we show that nest males decide about acceptance or expulsion of females solely by chemical cues of foreign larvae contained in their brood. Brood caring nest males experimentally exposed to chemical cues of their own broods and different numbers of foreign offspring rejected the broods based on the absolute number of foreign young, whereas relative numbers of own and foreign young, and the absolute number of own young had no effect. Male differentiation between chemical cues of own and foreign offspring occurred in the early larval stage, but not in the egg stage or with older larvae. The eviction of females can shorten the males’ interval to future reproduction. The described expulsion mechanism punishes females for the production of offspring with parasitic males, which may reflect an adaptive mechanism in species with alternative male mating tactics and external fertilization.