Platyphora leaf beetles form a vast group of tropical species each feeding on a restricted set of host plants and exhibiting bright coloration warning predators against their chemical protection. These beetles offer an exceptional opportunity for understanding the evolution of phytochemical sequestration. Indeed, qualitative studies of defensive secretions indicate that Platyphora species acquire toxicity via sequestration of plant secondary metabolites. All produce pentacyclic triterpene saponins from sequestered plant amyrins, but our analyses also indicate that many Platyphora species produce a dual chemical defence, that is, they are additionally protected by lycopsamine-type pyrolyzidine alkaloids that they also sequester from their host. This paper reports on the evolution of chemical defence and host affiliation in Platyphora leaf beetles as reconstructed on a molecular phylogeny of mitochondrial and nuclear DNA sequences. The analyses indicate that dual sequestration could be the key mechanistic means by which transitions among ecological specializations (i.e. restricted host-plant affiliations) are made possible.