Informal institutions governing the use of wild species are present in many societies. A system of prohibitions known as fady is central to Malagasy culture. We examined whether fady that relate to the use of natural resources in the eastern rainforests of Madagascar play an important conservation role. Prohibitions ranged from strict taboos in which a species or area was forbidden by the ancestors to social norms that concerned acceptable behavior when harvesting wild species. Strict taboos offered real protection to threatened species, such as the lemur Propithecus edwardsi and the carnivore Cryptoprocta ferox. Taboos also reduced pressure on some economically important endemic species by preventing their sale or limiting the harvest season. Despite their value for conservation, the taboos did not appear to originate from attempts to sustainably manage resources. Nevertheless, social norms, in which the sanction was social disapproval rather than supernatural retribution, encouraged sustainable harvesting practices for tenrecs (Tenrec ecudatus) and pandans (Pandanus spp.). Unfortunately, the social norms concerning methods of harvesting pandans appeared to be breaking down in villages surrounding Ranomafana National Park, and we suggest that the imposition of external conservation rules is weakening traditional management. Informal institutions are important to conservation because they suggest ways of improving cultural understanding and conservation communication. Food taboos influence societal preferences, which affect the wider demand for a species. Most important, where capacity to enforce external conservation rules is limited, informal institutions may provide the only effective regulations. Informal institutions should receive greater attention from conservation biologists so that local people's conservation roles can be acknowledged fairly and so that potential synergies with conservation objectives can be realized.