1. Parasitized females in mammals, fish and birds can enhance the immune defence of their offspring by transferring specific antibodies for the embryo. Likewise, social insect mothers transfer immunity despite the fact that invertebrates lack antibodies. 2. Female trans-generational immune priming is consistent with parental investment theory, because mothers invest more into rearing their offspring than fathers. However, when immune priming is not directly linked to parental care, as is often the case in insects that abandon their eggs after oviposition, both sexes might benefit from protecting their offspring. 3. Using the red flour beetle, Tribolium castaneum, we show that after parental exposure to heat-killed bacteria, trans-generational immune priming occurs through fathers as well as mothers. 4. This novel finding challenges the traditional view that males provide only genes to their offspring in species without paternal care, and raises the possibility of a division of tasks with respect to immune protection between parents.