The behavioural ecology of western lowland gorillas was studied for 16 months in the Lope Reserve, Gabon, where gorillas are sympatric with chimpanzees in lowland tropical forest. Data were collected by direct observation, and by examination of nest-sites, feeding-sites, and trails. The nature of frugivory and the extent of seasonal variation in food selection were emphasised. Dietary composition was identified, and the contribution of fruits was evaluated from the volume of fruit ingested estimated retrospectively from seeds in the gorillas' dung. Forest structure and composition were assessed using transects, and fruit and leaf production was quantified monthly to estimate food availability. Food distribution was patchy, and many foods showed seasonal peaks in abundance. The heterogeneity of the habitat was reflected in the diverse diet: gorillas ate 139 parts of 103 species of plants, including 78 fruits. One third of dung samples contained weaver ants. Vegetative parts of Aframomum and Marantaceae formed staple foods, due to their abundance, accessibility, and year-round availability. Succulent fruit formed over 90% of fruit intake. Seasonal variation was measured in all dietary parameters. Flexible foraging strategies enabled gorillas to cope with fruit scarcity, particularly during the major dry season: when less fruit was available gorillas consumed more stems, leaves, and bark, and ate poorer-quality fibrous fruits. Ranging was influenced by the seasonal availability of particular food species: when fruit was abundant gorillas travelled large distances between sources, when scarce they adopted a low cost strategy, shifting their diet towards more abundant, but poorer quality foods, and travelling less. Differences in feeding, ranging, and climbing between lowland and mountain gorillas result from striking differences in their respective habitats, especially in the abundance and distribution of fruit sources. Lowland gorillas' home ranges were larger; they spent more time in tress, mostly feeding; yet their social structure seemed to be similar to mountain gorillas. Lope gorillas adopted strategies similar to those of other frugivorous primates: fruits were preferred foods, consumed with fibre and leaves to meet nutritional requirements. The switch in diet was facilitated by the gorillas’ large body-size, which may have enabled them to cope with succulent fruit shortages, and allowed gorillas to remain in relatively stable groups.