From October 2004 to September 2005, I conducted a study on the behavioral ecology of the rare and endangered Río Mayo titi monkey (Callicebus oenanthe) in a fragmented premontane tropical forest of the Alto Mayo, near the town of Moyobamba, in northern Peru. This is the first such field study on this species. The study group consisted of an adult male, an adult female, and two offspring (a male and a female). An infant was born in May 2005. No offspring dispersed during the study. Data were recorded using instantaneous focal animal sampling and it was possible to obtain detailed information on behavior because the individuals could be identified and the group was easily followed at close range. There are pelage differences among the age and sex categories. Pelage coloration differences in individuals of different groups may indicate color morphs or possible subspecies. The conservation status of C. oenanthe is described, with the conservative estimate that it persists in less than 1,800 km² of forest, much of it now fragmented. Callicebus oenanthe is extremely threatened by deforestation, a severely fragmented habitat, and the scarcity of large forest tracts to maintain viable populations. I recommend that it should be upgraded to the status of Endangered in the IUCN Red List.