Saguinus mystax (Spix, 1823), a callitrichid (tamarin and marmoset family) commonly called the mustached tamarin, is 1 of 12 species in the genus Saguinus. In this large callitrichid (450–650 g), females are slightly heavier than males. It occurs in Brazil and Peru, south of Rio Amazonas from Río Ucayali in the west to Rio Purus in the east, in primary and secondary tierra firma rainforests. The diet consists of fruit pulp, insects, gums, nectar, and small vertebrates. It lives in groups of 3–10 individuals with a male-biased adult sex ratio and has a cooperative breeding system. In areas of sympatry, S. mystax forms mixed-species groups with saddle-back tamarins, Leontocebus. Although captured for the pet trade and sporadically hunted, it is considered “Least Concern” (LC) by the International Union for Conservation of Nature.