Protein is essential for living organisms, but digestibility of crude protein is poorly understood and difficult to predict. Nitrogen is used to estimate protein content because nitrogen is a component of the amino acids that comprise protein, but a substantial portion of the nitrogen in plants may be bound to fiber in an indigestible form. To estimate the amount of crude protein that is unavailable in the diets of mountain gorillas (Gorilla beringei) in Bwindi Impenetrable National Park, Uganda, foods routinely eaten were analyzed to determine the amount of nitrogen bound to the acid-detergent fiber residue. The amount of fiber-bound nitrogen varied among plant parts: herbaceous leaves 14.5±8.9% (reported as a percentage of crude protein on a dry matter (DM) basis), tree leaves (16.1±6.7% DM), pith/herbaceous peel (26.2±8.9% DM), fruit (34.7±17.8% DM), bark (43.8±15.6% DM), and decaying wood (85.2±14.6% DM). When crude protein and available protein intake of adult gorillas was estimated over a year, 15.1% of the dietary crude protein was indigestible. These results indicate that the proportion of fiber-bound protein in primate diets should be considered when estimating protein intake, food selection, and food/habitat quality. Am. J. Primatol. 70:690–694, 2008. © 2008 Wiley-Liss, Inc.