Forest fragmentation and isolation can reduce the size of available habitat and lead to lower food availability for some primate species. The persistence of nonhuman primates in fragments depends largely on their ability to adjust their diet in response environmental change. The western black crested gibbon (Nomascus concolor) is distributed in northern Vietnam, northwestern Laos, and southwestern China, but little is known about its diet except from studies in the well-protected forests of Mt. Wuliang and Mt. Ailao, central Yunnan. We studied food abundance and diet over 2 years in a small group surviving in an isolated and disturbed forest at Bajiaohe, southern Yunnan, and drew a comparison with the population at Dazhaizi in Mt. Wuliang. We found that gibbons at Bajiaohe consumed mostly fruit, but did not eat figs, unlike most other gibbon populations. Liana fruits and mature leaves were used as alternative foods during periods of tree fruit scarcity. Our results indicate that gibbons in Bajiaohe respond to habitat fragmentation and isolation by consuming a variety of plant species, depending on those that are locally available, and increasing time spent feeding on fruits of trees and lianas rather than increasing time spent consuming leaves. Am. J. Primatol. © 2013 Wiley Periodicals, Inc.