Stilobezzia enigma, a new Neotropical species, is described and illustrated from larvae, pupae and adults. Larvae and pupae were collected from internodes of two bamboo species, Guadua chacoensis (Rojas) and Guadua trinii (Ness) Ness ex Rupr (Bambuseae) in Misiones province, Argentina, and were examined in the laboratory with a scanning electron microscope. The generic placement of S. enigma is somewhat uncertain but we provide cladistic evidence indicating that it is probably an autapomorphic member of Stilobezzia.