The many island countries of the South Pacific region contain extensive areas of reefs and lagoons that provide highly favourable habitats for giant clam mariculture and related activities. These reef and lagoon areas fall almost entirely under customary forms of marine tenure whereby fishing rights are held by indigenous tribal, village or family groups. In general, these marine tenure arrangements confer upon members of each tribal and village group the right to share in the marine resources of the group, subject to any restrictions that may be imposed by custom. The customary marine areas that form the traditional fishing grounds of a tribal or village group are the usually adjacent reef and lagoon areas which extend from the land boundaries to the outer edge of the fringing reef. The sharing of marine areas over which a particular customary owner holds exclusive fishing rights is fairly common throughout the South Pacific region. A rich tradition of sharing in the exploitation of marine resources is still found among tribal and village groups. In general, this sharing is not restricted to members of a particular customary group but extends to outsiders, particularly neighbouring villages and an element of reciprocity is usually involved in these cases. As it currently operates among the Pacific island countries, customary marine tenure can be a significant constraint to the development of a major giant clam mariculture project. Uncertainty over ownership rights of customary groups can, for example, involve an unacceptable degree of risk for the prospective developer who will wish to locate his operations in a country and region where marine property rights favour success of the project. To gain access and lease rights over a section of reef and lagoon that is suitable for a giant clam mariculture project, the developer has to obtain the consent of the customary group which controls the area in question. Such consent is normally the prerogative of the tribal or village leaders as representatives of their people. From the available evidence, it seems that whether or not consent is given largely depends on how villagers perceive the kind of benefits expected to derive from a giant clam mariculture project. Perceived benefits appear to include only monetary payments but also those that can arise from a project's impact on local infrastructure, nutrition and the regeneration of fished-out reef and lagoon areas. Consent can also significantly depend on the involvement of local villagers in the development and operation of such the project, for example, by equity contribution, appointment to senior positions, and employment participation. Giant clam mariculture can be initiated and operated by villagers themselves and this approach may be most suitable in relation to small-scale, subsistence-orientated projects. For major commercial ventures the participation of outside developers seems imperative, especially to provide capital funds and technical knowhow. Various forms of institutional arrangements are possible, including joint ventures, but a vital prerequisite is to ensure significant and meaningful participation by the local people. Tribal and village authorities have a range of customary sanctions at their disposal which can be applied to protect a giant clam mariculture project site from encroachment by villagers. These authorities can also support such a project by assisting in the policing of the project site against both local and outside poaching.