A new study in Caenorhabditis nematodes has shown that the species barrier is maintained by highly aggressive sperm. Rather than producing infertile offspring as a result of interspecific mating, as observed in other species, interspecific mating in Caenorhabditis spp. results in maternal infertility and shortened maternal lifespan. Caenorhabditis hermaphrodites mate with more than one male and, during interspecific matings, aggressive sperm displace conspecific sperm and can decrease lifespan or cause death by invading other tissues. This study provides insights into how conflict over reproductive interests can lead to alternative modes of speciation and further the evolution of reproductive traits.