The polychaete annelid Paralvinella pandoraeDesbruyères and Laubier 1986 is endemic to hydrothermal vents in the northeast Pacific, and is found at almost all vents sites along the 500-km long Juan de Fuca ridge (JdF) system. The sperm morphology of P. pandorae indicates that fertilization occurs internally or in the worm's tube, and the maximum observed oocyte size of 215 μm suggests that a dispersive larval phase is short or non-existent. Size frequency analyses of populations of P. pandorae suggest continuous or semi-continuous recruitment of juveniles. Given our limited knowledge of the species’ life history, we predicted that populations of P. pandorae would exhibit a decline in genetic similarity with increasing distance among populations along the JdF. While our attempts to use amplified fragment length polymorphisms to test this prediction were not successful, our analysis of cytochrome oxidase I gene sequences provided insights into the phylogeography of the species. For 31 individuals from five sites along the JdF there is little sequence variation among individuals and no phylogeographic pattern among haplotypes from populations separated by distances of up to 210 km. These results indicate that gene flow occurs among all sites in the analyses, i.e. despite the very limited dispersal potential inferred from life history characteristics of this worm, there is no evidence for isolation-by-distance across the geographical scale of the study. Demersal larvae dispersed by near-bottom currents might explain the gene flow among sites, as well as the establishment of populations of P. pandorae at new vents within a year.