The biogeography and evolutionary history of animals that live at hydrothermal vents are connected intimately to the spreading history of mid-ocean ridges. Extensive collections from two active ridge systems in the eastern Pacific Ocean provide an opportunity to examine the regional dispersion of vent-limited organisms. The degrees to which these habitat-limited species from disjunct areas are related gives preliminary information about exchange routes, dispersability, and rates of taxonomic change. Differences between vent faunae from the northern Juan de Fuca and southern East Pacific Rise systems indicate that geographical differentiation has occurred. Geophysical evidence shows that North America interposed as a barrier between the northeast and equatorial Pacific spreading ridges about 35 Ma BP. The vicariating vent fauna of the Juan de Fuca Ridge has since formed an endemic assemblage of generally lower diversity than that found at East Pacific Rise vents. Taxonomic comparisons suggest that rates of speciation have been low. Examination of spreading histories elsewhere should provide predictions of evolutionary patterns in the hydrothermal-vent faunae.