On 13 April 2007, during a preliminary field survey of the current distribution and conservation status of the yellow-tailed woolly monkey (Oreonax flavicauda) in the departments of San Martin and Amazonas, Peru, we encountered a group of eight O. flavicauda consisting of five adults and three juveniles. The encounter took place in a privately owned, unprotected forest at an elevation of 1900 m a.s.l., 1 km NW of the village of Santa Rosa (05°40′13.5″S, 77°55′08.0″W). This area is highly disturbed primary montane forest interspersed with pasture. The group was followed for approximately one hour. During the entire encounter we observed the presence of a female whitebellied spider monkey (Ateles belzebuth) freely associating with all members of the O. flavicauda group. The female spider monkey continued to travel with the group as they entered the next valley, but we were unable to follow. Both species showed complete tolerance to each other and an equal intolerance of the presence of humans, with loud vocalizations and branch shaking. Observation was made easier by the aggressive approach of the group toward us. We believe this is the first record of such an association and highlights the need for further detailed study of both species in the wild, in particular the critically endangered yellow-tailed woolly monkey about which we know so little.