Diet and ranging patterns of gorillas (Gorilla gorilla graueri) and chimpanzees (Pan troglodytes schweinfurthii)were studied during the 1994 dry season in the montane forest of KahuziBiega National Park. A transect census was also made to estimate the diversity and density of trees in primary forest, secondary forest and swamp. Although the diversity of trees in this area was lower than those estimated in lowland tropical forest, more than half of the total 50 tree species were used for foods either by gorillas or by chimpanzees during the study period. A semi-habituated group of gorillas and a unit group of chimpanzees extensively overlapped their diet and ranging areas, but gorillas tended to consume fibrous foods qualitatively and quantitatively and to range in wider area than did chimpanzees. Gorillas daily traveled in both primary and secondary forest to avoid reuse of the previous ranging areas, while chimpanzees persistently range in a small patch of primary forest. The diversity and density of food trees with bearing fruits were higher in primary forest than in secondary forest, and both apes preferred the fruit species which trees appeared at low densities in the transect. The low density of chimpanzees may be owed to the small size and fragmented distribution pattern of primary forest in this area. The differences in foraging patterns between gorillas and chimpanzees may reduce ecological competition between them.