Rough-skinned newts produce tetrodotoxin (TTX), which is a deadly neurotoxin that blocks voltage-gated sodium channels in predators. However, so far the origin of the toxin was not clear. Here, Eisthen and colleagues tested whether highly toxic newts harboured TTX-producing bacteria. They cultured isolates of the skin-associated microbiota from a toxic population and a non-toxic population of newts and identified TTX-producing bacterial strains from four genera in the collected samples, including Aeromonas, Pseudomonas, Shewanella and Sphingopyxis. This observation suggests that the symbiotic bacteria are the source of TTX toxicity in those animals. Finally, the authors showed that TTX resistance in toxic newts and other animals was owing to modified TTX binding sites in the sodium channel, which indicates a role of the host-associated bacteria in shaping the evolution of TTX resistance.