AbstractMuch information has been published concerning the accurary of various techniques commonly used for censusing primates. The sources of bias may vary according to the technique used. Accuracy may depend on population density as well as on differential visibility and hence detectability between areas. This paper provides evidence that primates may alter their behavior between areas of primary and selectively logged habitat, which in turn could affect census results. Adoption of cryptic behavior, changes in behavioral profile, and reduction in calling rates reduce the detectability of certain species. Increases in the frequency of subgrouping or in group dispersion may introduce a danger of overestimation of other species, unless differences in the size of foraging units are taken into account.