Over an eight-year period, a total of 174 food items were recorded for chimpanzees (Pan t. troglodytes) in the Lopé Reserve in central Gabon. Plant foods, principally fruit, dominated the diet but insects were eaten regularly, and predation on at least three species of mammal occurred infrequently. The diversity of the vegetative component of the diet (leaves, stems, and bark) was probably underestimated by fecal analysis. Comparison of chimpanzee diet at Lopé with that of sympatric lowland gorillas showed the majority of foods were eaten by both species (73% of chimpanzee food items and 57% of gorilla food items). The overlap of fruit species was greater (82% and 79%, respectively) than that of other food classes. Both chimpanzees and gorillas harvested the majority of their plant foods arboreally (76% and 69%, respectively). The high degree of dietary overlap suggested that ecological competition between these two closely related species might exist. Few overt signs of competition for food either between or within species were observed but when fruit was scarce, the diets of the two species showed greatest divergence. The major differences between chimpanzee and gorilla diet at Lopé were the larger quantities of vegetative foods regularly eaten by gorillas and their ability to resort to a diet dominated by vegetative foods when fruit was scarce. In these respects, chimpanzees at Lopé resembled populations of Pan troglodytes studied elsewhere while Lopé gorillas resembled mountain gorillas (Gorilla g. beringei) and bonobos (Pan paniscus) in their greater dependence on vegetative foods. © 1993 Wiley-Liss, Inc.