The new fruticose lichen species Trapeliopsis studerae with fruticose growth form, branches mostly flat to partly curling up, but lower down often almost cylindrical, much irregularly branched, 0.1– 0.2 mm thick and 0.3–0.6 mm wide, upper surface olive grey, pycnidia that probably belong to the lichen are present on some older parts of the thallus, superficial, brown, conical to somewhat inflated, conidia hyaline, clavate, 8–1131.5–2.5 lm, and chemistry gyrophoric acid, is described from rock outcrops in a remnant of Atlantic Forest in Alagoas, Northeast Brazil, viz. Reserva Bi ´ ologica de Pedra Talhada, near the city of Quebrangulo, in the middle of the semiarid region in Northeast Brazil. It represents a new and independent lineage of fruticose lichens, the first found in the Trapeliales and only the second in the subclass Ostropomycetidae. The morphology of this species is enigmatic: it somewhat resembles a Siphula or a Stereocaulon, but it is irregularly branched without main stem, lacks cephalodia and apothecia, and it differs from all known species in these genera by the gyrophoric acid chemistry. It forms dense mats on siliceous rock that is influenced by run-off water. It typically grows at the upper ends of gullies that are occupied lower down (where there is more often water) by cyanophilic lichens such as Peltula clavata and Jenmania osorioi. The habitat is extremely poikilohydric; this lichen is occasionally totally submerged, but usually completely dry. Keywords: Atlantic Forest, Stereocaulon, Ostropomycetidae, Trapeliopsis, poikilohydric.