This paper examines the coastal ejido (collectives of peasant landholders) sector's response to the 1992 amendment to Article 27 of Mexico's Constitution, which for the first time legalized the sale and rental of ejido lands. Our analysis is based on a case study of southern Sonora, Mexico. The results indicate a shift in land tenure from the ejido to the private sector and a corresponding conversion of coastal lands to shrimp aquaculture ponds. Our analysis suggests these land-tenure and land-use changes, and the implications of these changes for the coastal ejido sector, vary depending on the historical, geographic and socio-economic characteristics of the ejido communities.