Many tropical nature reserves are woefully understaffed or exist only on paper. Without effective implementation, tropical reserves cannot count on in situ enforcement and consequently are subject to a wide range of invasive threats. Weak institutional structures are aggravated by reserve designs that facilitate rather than discourage unlawful human activities. Taking into account severe financial and institutional constraints, we consider the current status of forest reserves in lowland Amazonia. We ask how the criteria by which reserves are delimited may affect the efficiency with which the contained areas are defended. In a GIS analysis, we found that 40 to 100% of the area of all existing nature reserves in Brazilian Amazonia are directly accessible via navigable rivers and/or functional roads. Such access greatly facilitates the illegal harvest and conversion of forest resources in a region where each guard is responsible for protecting an area larger than the State of Delaware. Cost‐effective defense of large areas can be achieved through appropriate delimitation of reserves along watershed divides and by efficient deployment of limited infrastructure and personnel. Given current and probable future levels of financial resources allocated to reserve maintenance in Amazonia, any new nature reserves in this region should be designed and situated so that their defensibility is maximized. Defensibility criteria should complement site considerations based on biological criteria, such as presumed centers of diversity and endemism. Copyright © 1995, Wiley Blackwell. All rights reserved