Traditional Central American peasant farmers know more about some aspects of the local agroecosystem than about others. In general farmers know more about plants, less about insects, and less still about plant pathology. Without discounting economic factors, ease of observability must explain part of this difference. Certain local beliefs may affect what farmers observe and know. For example, a belief in spontaneous generation may lead people to fail to observe insect reproduction. The implications of the gaps in farmer knowledge are discussed in terms of the sustainable agriculture movement.