Rhinopithecus bieti, a foregut-fermenting colobine monkey, is one of the few animals in the world that subsists largely on lichen for much of the year. Lichen is rarely consumed as the primary food source by most animals for very long because of its low protein and tendency to contain anti-nutritive compounds. Samples of Usnea longissima collected in R. bieti habitat depressed in vitro fermentation. In a selection trial, wild monkeys showed strong selection for one Usnea chemotype over a second type. The partial dry matter digestibility of Usnea longissima fed to captive R. bieti that were also fed other foods was high (81.3 ± 3.5%, n = 5). Endogenous fecal protein losses were estimated at 2.8 ± 1% of dry matter intake, and true protein digestibility was 88 ± 6%. Although typically primates select diets with high protein: fiber ratios, R. bieti fed high-lichen diets did not follow this trend, instead selecting against lichen and for a diet with a lower protein:fiber ratio. R. bieti relies on lichen as a staple fallback food during harsh winters, and is able to do so despite lichen's antimicrobial properties. R. bieti may be in or near negative nitrogen balance in winter if they consume a lichen-only diet, and this will be most pronounced for growing animals or pregnant or lactating females. Because lichens are so sensitive to climate change, and R. bieti habitat is expected to have one of the most rapidly-changing climates on earth, careful monitoring of not only lichen abundance, but also the chemotypic composition of lichens is needed in order to evaluate the sustainability of R. bieti in their current habitat.