This is an 18-year study of the endangered Papilio (Pterourus) homerus, adding substantial information to our scanty knowledge of its ecology. The contraction of a once contiguous but narrow population on a single Caribbean island carries the serious threat of extinction. There are now two populations or probably metapopulations, effectively isolated from each other. The butterfly’s larvae feed on Hernandia catalpaefolia and H. jamaicensis, both endemic to Jamaica, and development takes ∼84 days from egg to the emerged adult. Adult numbers fluctuate rapidly, with peaks in July/August each year. Egg distribution was studied at three spatial levels: the food item (leaf cluster), the patch (tree) and the habitat (each valley). Major causes of developmental mortality were Chrysonotomyia, a eulophid parasitoid of the eggs, and bacterial infection of the larvae and pupae. Critically, the mortality from this wasp was lower in undisturbed forest than in the area disturbed by agriculture, this finding having important consequences for conservation. Although there was no evidence of a decline in numbers over the last century, we believe this is an artefact due to collectors working only at the periphery of its distribution. Even assuming that its population densities have not changed, the contraction of its usable habitat implies a similar reduction in average numbers and the small populations are susceptible to disaster. The efforts of researchers, NGOs, and Government agencies have greatly increased the level of awareness, making the people in some key areas the ‘protectors of the species’.