A group of Tana mangabeys (Cercocebus galeritus) has greatly expanded its annual range over three decades: from 17.25 hectares (ha) in 1974 (Homewood  Ph.D. dissertation, University College London) and 18.75 ha in 1988–1989 (Kinnaird [1990a] Ph.D. dissertation, University of Florida) to 46.75 ha in 2000–2001. Utilizing 5-min mapping samples, phenological samples, vegetation data, and statistical and descriptive comparisons with previous data, three hypotheses concerning this range increase were postulated. The hypothesis with the strongest support is an increase in intragroup feeding competition, which was measured by a decrease in fruit availability per hectare and an increase in group size. Fruit biomass estimates of four species individually and of eight species combined showed significant decreases between earlier findings (Kinnaird  Ph.D. dissertation, University of Florida) and this study. Group size decreased from 36 in 1974 to 17 in 1988–1989, and is now 50 individuals. There is no support for the hypotheses that the group is no longer constrained in home range size by neighboring groups or by the extent of forest cover. The changes in group and home range size are also discussed in the context of historical forest loss in the lower Tana River. The ability of the Tana mangabey to increase its home range, especially by traveling through nonforest habitat, is an important aspect of its ecological flexibility in a fragmented and threatened habitat. Am J Phys Anthropol, 2005. © 2005 Wiley-Liss, Inc.