Line-transect surveys were conducted at the Isecheno study site in the Kakamega Forest, western Kenya to estimate diurnal primate densities. The estimates from several different methods of analysis of census data were compared to "true" density values based on home range size and overlap for two species. The Whitesides method [Whitesides et al., 1988], which incorporates species-specific mean group spread into its formula for estimating transect width, provided the most accurate density estimates. The importance of including as many groups as possible when calculating density from home range size and overlap is demonstrated with long-term data from Colobus guereza and Cercopithecus mitis. Colobus guereza group density at Isecheno was much lower than that published from a recent brief study [von Hippel, 1996]. Cercopithecus mitis group density has fallen while overall population biomass appears to have remained stable over 20 years of study. Isecheno has the second highest diurnal primate biomass of the ten Guineo-Congolian rainforest sites for which biomass data are available, despite having the lowest primate species richness. Within the Guineo-Congolian rainforest system, primate biomass appears to vary to some extent between ecogeographic regions: two of three mid-elevation East African sites have high biomasses, two of two lowland West African sites have intermediate biomasses, and four of five lowland Central African sites have low biomasses. There is a strong positive correlation between total colobine biomass and total primate biomass at the ten Guineo-Congolian rainforest sites.