This paper presents a method for estimating monkey numbers in a large area of forest where there is a gradient of monkey densities. The method is illustrated using data collected in the northeastern forests of Gabon during an earlier project. These forests are sparsely populated and there are few roads. The density of Cercopithecus nictitans increases with distance from the nearest road. A geographic information system (GIS) divided the forest into bands of increasing distance from the nearest road. The number of monkeys in each band is the product of the monkey density in that band and the area of the band. Summing across bands gives the population estimate; the standard error can be estimated by bootstrapping. The optimum sample size can be estimated by simulation. Combining estimates of the density gradient with a GIS is a cost-effective method of censusing primates in extensive forests.