Abstract
Peru has the highest diversity of members of the tamarin genus Leontocebus (Callitrichidae). However, for a number of taxa from this genus the distributional ranges are still not well known. In this paper we provide evidence for the extension of the southern range of Leontocebus fuscicollis fuscicollis to the right bank of the Río Abujao, south of which it is replaced by Leontocebus weddelli weddelli.