THE Vestimentifera are gutless worms that live around deep-sea hydrothermal vents and cold seeps, obtaining energy from hydrogen sulphide with the aid of endosymbiotic chemosynthetic bacteria(1-3). Their phylogenetic relationships have been debated ever since they were first discovered(4,5). Moreover, hydrothermal vents are ephemeral and spatially patchy, raising questions about how vestimentiferan populations are established and maintained(6-9), and how symbionts are transmitted(10). Although postsettling juveniles have been described(11,12), embryos and larvae have been neither collected nor cultured. Here we describe the early development of vestimentiferans from cold seeps in the Gulf of Mexico(13), and discuss the implications of our findings for dispersal potential and phylogeny.