Thallus: erect-shrubby, 2 to 5 cm long branching: anisotomic dichotomous, divergent basal part: concolorous with the branches, not distinctly annulate branches: cylindrical to tapering, not distinctly segmented; lateral branches: not narrowed at attachment points segments: terete to distinctly ridged, cylindrical tubercles: absent fibercles: absent to few fibrils: short (1-2 mm), conspicuous, spinulous, not easily broken away, usually densely and regularly disposed all along the branches, giving a spinulous appearance to the whole thallus isidiomorphs: absent cortex: shiny, not distinctly cracked, moderately thick (5-7%) medulla: dense to compact, not pigmented axis: moderately thick, not pigmented Apothecia: numerous, up to 5 mm in diam., mainly terminal ascospores: broadly ellipsoid, 9-10 x 5-6 µm Spot tests: medulla K-, C-, KC-, P- Secondary metabolite: diffractaic (major). Substrate and ecology: on bark in thorn forest with Acacia, Lysiloma and Jatropha. World distribution: mainly eastern and southeastern North America, rare in Mexico Sonoran distribution: southern Baja California. Notes: Usnea strigosa has many different chemotypes and needs a modern and thorough revision of its taxonomy. It is characterized by its tufted habit with numerous terminal apothecia, its densely and regularly disposed fibrils that are short and spinulose, its ridged branches that are not narrowed at the point of attachment, the absence of papillae and its basal part that is not black pigmented. All the Mexican specimens studied have an unpigmented medulla. This diffractaic acid chemotype is so far the only chemotype that has been found in Mexico. Usnea parvula is morphologically very close to U. strigosa but the former species has shorter spores, a thinner cortex, typical foveoles, as well as a lax medulla without diffractaic acid.