Mantled howler monkeys (Alouatta palliata) occupy a wide variety of tropical habitats and are the most folivorous of New World primates. However, their diet may include fruits, buds, petioles, and flowers, as well as leaves, suggesting they must cope with variations in the nutrient composition of their food. We studied the physiological basis of the dietary flexibility of these monkeys by comparing food choice, digestive performance and patterns of digesta flow in six adults, fed diets of either leaves or a mixture of fruit and leaves. Although monkeys ate similar amounts of the two diets, they ingested more digestible protein when offered the leaf diet, on which they lost body mass, but they ingested much more soluble sugars when offered fruit and leaves on which they gained mass. Digestibilities of dry matter, fat, energy and fibre did not differ between diets, but those of crude protein, soluble sugars and minerals were higher on the fruit-leaf diet. Mean retention times in the gut of solute (Co-EDTA) and particulate markers (Cr-mordanted cell walls) did not differ between diets, but on both diets the monkeys retained the particulate marker (mean retention time ca 55 h) for longer than they did the solute marker (MRT ca 50 h). A lack of selective retention of solutes and small particles in the gastro-intestinal tract of howler monkeys probably restricts them to mixed diets but their digestive strategy is sufficiently flexible to allow them to feed on a diet of leaves when fruit is unavailable.