Thallus: fruticose, pendulous, up to 30 cm long branching: subparallel, irregularly or dichotomously branched, growing from a narrow holdfast branches: solid, mostly flat in basal branches, two-edged in distal ones, 1-3 mm wide surface: yellowish green, smooth or conspicuous striation by pseudocyphellae, shiny, without soredia pseudocyphellae: common, laminal or rarely marginal, narrowly linear cortex: thin; chondroid strands: discontinuous, heavily cracked Apothecia: common, stipitate disc: flat to +concave, without white margin; margin: concolorous with the thallus asci: elongate-clavate, 8-spored ascospores: hyaline, 1-septate, narrowly fusiform, 18-25 x 3.5(-4) µm Pycnidia: not observed Spot tests: cortex K-, C-, KC+ yellow, P-; medulla K-, C-, KC-, P- Secondary metabolites: cortex with usnic acid; medulla with sekikaic (major) and homosekikaic acid (major) and 4'-O-methylnorhomosekikaic acid (minor) for Sonoran specimens. Substrate and ecology: on bark World distribution: southern North America, the Caribbean and Central and South America Sonoran distribution: only few collections in Sinaloa, more common in southern states of Mexico at elevations between 800 to 1200 m. Notes: Although several chemical races of R. usnea have been reported (Kashiwadani 1993; Landrón 1972), only sekikaic acid aggregate race are found in the Great Sonoran region. Ramalina usnea resembles R. sharpii Rundel, a species reported from Hidalgo, Mexico, that differs in having smooth chondroid strands, broadly fusiform ascospores and norstictic acid. Ramalina usnea might be confused with R. menziesii that differs in having fenestrate branches and in lacking medullary chemical metabolites. It also resembles one morphology of R. alludens with marginal, continuous pseudocyphellae. However, it can be distinguished by its narrowly linear pseudocyphellae and by its narrowly fusiform ascospores; R. alludens has broadly fusiform ascospores.