Primate diets may be influenced by the phytochemical composition of potential food resources. This study examined the protein, fiber, and mineral content of leaves from plant species used as food resources by the proboscis monkey (Nasalis larvatus) in Tanjung Puting National Park, Kalimantan Tengah, Indonesia. In addition, leaf samples were collected from nonfood resources based upon relative density in previously established vegetation plots. Higher concentrations of crude protein, phosphorus, potassium, and zinc were found in young leaves. Mature leaves had higher levels of magnesium, manganese and calcium compared to young leaves. Leaves from species used as food resources were significantly higher in protein (9.9% vs. 8.5%) and lower in fiber (30.5% vs. 40.0% acid detergent fiber) than the leaves from species not used as food sources. Food resources also contained higher concentrations of phosphorus and potassium; nonfood resources contained higher concentrations of calcium and manganese. Proboscis monkeys appear to follow the typical colobine trend: a diet relatively high in protein and low in digestion inhibitors (fiber). Little previous work has examined mineral content as a selection factor in primate food choices. Leaves from species used as food resources contained less calcium (0.35%), iron (34 mg/kg) and phosphorus (0.15%) than dietary standards established for captive nonhuman primates, but values were similar to those established for ruminant herbivores. Other mineral concentrations (potassium, magnesium, manganese, sodium, and zinc) met or exceeded recommended levels for both primates and ruminants. Comparison of diverse nutrients in native diets may provide insight into suitability of domestic animals models for providing adequate diets in captive feeding situations, as well as help define resource limitations in natural environments.