We examined seasonal patterns of fruit availability dietary quality and group size in the descendants of an introduced chimpanzee population on Rubondo Island Tanzania. The site has supported a free-ranging population without provisioning for 40 years. Our goals were to determine whether Rubondo chimpanzees experience periods of fruit shortage and whether they respond to changes in fruit availability similarly to chimpanzees at endemic sites. We indexed the fruit availability of tree and liana species on transects stratified across three chimpanzee ranging areas. We used fecal analyses to evaluate seasonal changes in diet and used data on party size and nesting group size to examine seasonal patterns of grouping. Tree fruit availability was positively correlated with rainfall with a period of relative tree fruit scarcity corresponding with the long dry season. Liana fruit availability was not related to rainfall and lianas exhibited less variable fruiting patterns across seasons. Fruits made up the majority of the chimpanzee diet with lianas accounting for 35% of dietary fruit species. Fruits of the liana Saba comorensis were available during all months of phenological monitoring but they were consumed more when tree fruit was scarce suggesting that Saba comorensis fruits may be a fallback food for Rubondo chimpanzees. There were no increases in consumption of lower-quality plant parts between seasons and there were no changes in group size between seasons. These results contrast with evidence from several endemic chimpanzee study sites and indicate that Rubondo chimpanzees may have access to abundant and high-quality foods year round. Am. J. Primatol. 69:487-502 2007.