Abstract Trachypithecus delacouri (Osgood, 1932), commonly called Delacour's langur or lutung, is a black and white lutung (leaf monkey) endemic to northern Vietnam. It is a diurnal and primarily arboreal species, but spends more time on the ground than other species of Trachypithecus. T. delacouri lives among limestone cliffs and consumes the leaves of a broad spectrum of plant species but its choice of food items tends to be based on the chemical makeup of the plant phenophases with protein content the strongest predictor of leaf selection. Territorial, but with overlapping territories, it usually lives in single-male–multifemale groups of 5–30, although 2 adult males may be present. With a declining population perhaps as low as 200 individuals in 17 or fewer isolated populations, it is 1 of the world's most endangered primates.