There appears to have been little consideration given to the concept of marine reserves in open-ocean ecosystems, whereas a large number of protected areas for coastal marine and estuarine ecosystems have been or are now being developed around the world (Agardy 1994; Rowley 1994). We argue that, given the rapidly rising pressures on the oceans due to human overpopulation and associated resource extraction, conservation and management in the twenty-first century should include an international system of marine reserves for the open ocean. Such a system would be practical, and the benefits would be of global importance. We define open as including both the water column (the realm) and the sea floor (the benthic realm) in international waters, that is beyond 200 miles (320 km) from a nation's coastline. We have little doubt that there are long-buried and passing references in scattered literature to some aspects of the concepts we present. For example, de Fontaubert et al. (1996) note without further elaboration that certain ecosystems embraced by coastal marine protected areas might extend across legal national boundaries and into the high seas. Similar statements can probably be found in the burgeoning literature of marine conservation, biodiversity, and coastal marine reserves, but we have not found a distinct argument or proposal for reserves on the open ocean. Why should there be reserves on the high seas? Midocean waters remain largely without international environmental protection. The U.S. National Research Council (1995) identified the pelagic open ocean and the