The Matses Indians of northeastern Peru recognize two linguistically labeled folk species of saki monkeys (Cebidae: Pithecia) that are said to be morphologically distinguishable, to prefer different habitats, and to be noninterbreeding. Because the systematic literature recognizes only one biological taxon of Pithecia in the area inhabited by the Matses, their folk taxonomy suggests either the presence of a previously undescribed species or a hitherto undocumented range extension. All known species of Pithecia are currently thought to be allopatric, so both possibilities are of interest. We obtained blood samples and anatomical voucher material from hunters' kills in order to test the biological basis for the Matses folk taxonomy of sakis. Molecular and morphological analyses of the collected material indicate that both Matses folk species correspond to one effectionely panmictic population of Pithecia monachus. Overdifferentiation in folk classification systems, whereby one biological species is represented by two or more nonsynonymous folk species names, is a potentially widespread source of error that should be considered by researchers using local informants for primate field studies. In addition to resolving a folk-taxonomic enigma, our study provides the first quantitative analyses of local (within-population) morphological and molecular variation in this little-known platyrrhine genus.