Feeding trials were conducted with a troop of six proboscis monkeys (Nasalis larvatus) at the New York Zoological Park to quantify aspects of digestive physiology. Diets consumed comprised, on an as-fed basis, 55% browse and green produce, 11% fruit, 9% commercial primate diets, 9% root vegetables, and 16% miscellaneous items. Total feed intake averaged 12% of body mass; dry matter intake was 3%. Dry matter and plant cell wall disappearance exceeded 80%. Passage marker studies (three 6-day trials) revealed: transit time=14 hr; mean passage time=49 hr; and 5–80% retention time=52 hr. Digestion coefficients and retention times were greater than expected based on body size. These results might reflect the low fiber content of the diets fed, or alternatively, provide evidence that proboscis monkeys are efficient digesters of cell wall constituents.