Primate populations were surveyed quantitatively at three sites in the Xingu-Tocantins interfluvium in southeastern Amazonia, the easternmost limit of the distribution of the red-bellied titi, Callicebus moloch. At least 101 km was walked at a given site, and total transect length was 812 km. Despite the typical abundance of other species, no sightings (or vocalizations) of C. moloch were recorded at any site. This contrasts with other studies in the same interfluvium, and surveys in other areas of southeastern Amazonia. While the determinants of the absence-or marked rarity-of C. moloch from the sites remain unclear, it does appear to be a natural phenomenon, possibly related to specific local conditions. The identification and evaluation of such determinants will be important for understanding the ecology and zoogeography of the genus. In the meantime, these results highlight potential problems for the conservation and management of wild populations, not only of titis, but possibly many other species of Amazonian primates.