A survey of diurnal primate populations was carried out in Odzala National Park, Republic of the Congo, over 3 months in 1994 and 2 months in 1995. The park contains a high diversity and species-specific abundance of primates, and has the highest number of diurnal primates (10 species) in the forest block of Central Africa. Eight species of monkey: greater whitenosed monkey Cercopithecus nictitans, moustached monkey Cercopithecus cephus, crowned guenon Cercopithecus pogonias, De Brazza's monkey Cercopithecus neglectus, talapoin Miopithecus talapoin, white-cheeked mangabey Cercocebus albigena, agile mangabey Cercocebus galeritus and guereza Colobus guereza, as well as gorilla Gorilla g. gorilla and chimpanzee Pan t. troglodytes were sighted in the survey. Monkey species richness was highest in dense inundated forest and thicket, with all eight species occurring in these habitats, whereas only four species were found in terra firma forest (consisting of the park's two main habitats, open-canopy Marantaceae forest and closed-canopy primary forest). Three of the four species (C. nictitans, C. cephus and C. albigena) present in terra firma forest were most abundant in closed-canopy primary forest (1.4,1.0 and 0.6 groups per km, respectively) while the fourth (C. pogonias) was most abundant in open-canopy Marantaceae forest. Gorilla nests were most abundant in open-canopy Marantaceae forest (12.1 nests per km), while chimpanzee nests were mostly found in closed-canopy primary forest and Marantaceae forest (14 and 12 nests per km, respectively). Odzala has the highest recorded densities of western lowland gorilla (mean = 5.4 individuals per sq km) and chimpanzee (mean = 2.2 individuals per sq km) in Central Africa. The high densities of gorillas and chimpanzees may be a result of the high productivity of the forest and low poaching pressure. Conservation measures to ensure the maintenance of conditions in the area, such as educational programmes, are suggested.