In a collaborative study by ProNatura Chiapas, Defensores de la Naturaleza, Conservation International and CSF, we analyzed a dam proposed on the Usumacinta River in Mexico. Our objective is to stimulate discussion on the costs and benefits of such projects in Mesoamerica's largest watershed. We chose to analyze the Tenosique project (formerly known as Boca del Cerro), given that it is apparently the dam being given the most serious consideration by planners.