Data on the ecology and diet of buffy sakis (Pithecia albicans) were obtained during a 20-month study in an entirely undisturbed terra firme forest in the upper Urucu river, Amazonas, Brazil. Groups of 3–7 sakis found in a 900-ha study plot used large home ranges (147–204 ha), which overlapped extensively with those of neighboring groups. Similar to other pitheciines, buffy sakis were primarily seed predators, relying heavily on young seeds of certain key plant families, such as the Sapotaceae and Leguminosae. Ripe fruits, ripe seeds, young leaves, flowers, and nectar were eaten to a lesser extent. Whether or not feeding, sakis spent most of their time in the canopy and subcanopy, a pattern similar to that of other southwestern Amazonian saki species, but sharply different from that of Guianan sakis (Pithecia pithecia), which use considerably lower levels of the forest. Comparisons are made between different Pithecia species to show whether and how P. albicans diverges ecologically from its congeners. © 1993 Wiley-Liss, Inc.