This is a preliminary study on the comparative microscopic anatomy of the external noses of platyrrhine monkeys. Platyrrhinism is caused by the lateral width of the internarium (Alar cartilages, soft tissue in the sulcus interalaris covered by the integument). The septum nasi, situated caudal to the internarium is not involved in platyrrhinism. Aotus and Alouatta are, according to the definition, truely platyrrhine. The division of the higher primates into Platyrrhina and Catarrhina is justified. The anterior surface of the internarium (sinus hairs) is a sensorial area.