We describe the distribution and estimate densities of Grauer's gorillas (Gorilla gorilla graueri) and eastern chimpanzees (Pan troglodytes schweinfurthi) in a 12,770-km2area of lowland forest between the Lowa, Luka, Lugulu, and Oku rivers in eastern Democratic Republic of Congo, the site of the largest continuous population of Grauer's gorillas. The survey included a total of 480 km of transects completed within seven sampling zones in the Kahuzi-Biega National Park lowland sector and adjacent Kasese region and approximately 1100 km of footpath and forest reconnaissance. We estimate total populations of 7670 (4180–10,830) weaned gorillas within the Kahuzi-Biega lowland sector and 3350 (1420–5950) individuals in the Kasese survey areas. Within the same area, we estimate a population of 2600 (1620–4500) chimpanzees. Ape nest site densities are significantly higher within the Kahuzi-Biega lowland sector than in the more remote Kasese survey area in spite of a significantly higher encounter rate of human sign within the lowland sector of the park. Comparison of our data with information obtained by Emlen and Schaller during the first rangewide survey of Grauer's gorillas in 1959 suggests that gorilla populations have remained stable in protected areas but declined in adjacent forest. These findings underscore the key role played by national parks in protecting biological resources in spite of the recent political and economic turmoil in the region. We also show that forest reconnaissance is a reliable and cost-effective method to assess gorilla densities in remote forested areas.