Photorhabdus bacteria live in a 'symbiosis of pathogens' with nematodes that invade and kill insects. Recent work has begun to use the power of the model insect Drosophila to dissect the molecular basis of the invertebrate immune response to the combined insult of the worms and their symbiotic bacterial pathogens. By using RNA interference, it is now also possible to dissect this complex tripartite interaction in a range of both model and non-model hosts.