Diet, home range and day range of gorillas and chimpanzees that coexist sympatrically in the Kahuzi-Biega National Park are described, from a field study in 1994-96. A habituated group of gorillas and a semi-habituated unit-group of chimpanzees were followed on daily basis for collecting data on diet and ranging. Gorillas show a similar diet breadth to that of chimpanzees in the montane forest of Kahuzi. Fruits constitute an important part of their dietary composition and overlap extensively between them. However, seasonal changes in consumption of fruit are different between them. Gorillas prominently increased consumption of both diversity and amount of fruits during the dry season, while chimpanzees persistently consume fruits throughout the year, and the diversity and amount of fruits consumed by chiwpanzees does not correlate with each other. Gorillas had larger home range more than twice than chimpanzees, and used a wide area evenly. By contrast, chimpanzees tended to use the small area repeatedly. Gorilla’s ranging patterns are similar to those of herbivores contributing to regeneration of terrestrial herbs, whereas chimpanzee’s exhibited a typical frugivorous ranging patterns, which may take a role of seed dispersal. When consumption of fruits declined, gorillas decreased day range to reduce range, but chimpanzees tended to disperse. These differences in their foraging patterns during the period of fruit scarcity may support their sympatry by mitigating feeding competition.