Thallus: foliose, 3-6 cm in diam., adnate, rather fragile, subdichotomously lobate lobes: sublinear to subirregular, rather short, plane to subconvex, contiguous or becoming imbricate, 1-2 mm wide; margin: entire or dentate-crenate, sometimes lacinate; apices: subtruncate upper surface: pale gray-green, smooth, dull but shiny at the lobe apices, weakly white maculate, sometimes marginally pruinose lobulae: abundant, laminal and marginal, dorsiventral, tips, simple to forked, procumbent to subascending, with or without isidial inititials; soralia, pustulae and dactyls all absent medulla: white with continuous algal layer lower surface: black centrally to margin; rhizines: moderately abundant, black, richly dichotomous Apothecia: rare, laminal, sessile, up to 3 mm in diam.; disc: pale brown to cinnamon-brown; margin: smooth, sometimes lobulate, epruinose asci: clavate, 8-spored ascospores: subglobose, 5 x 4 µm Pycnidia and conidia: not seen Spot tests: upper cortex K-, C-, KC-, P-, UV-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with isousnic acid; medulla with echinocarpic acid and conechinocarpic acid (both minor). Substrate and ecology: on bark, in open pine-oak forests World and Sonoran distribution: thus far only known from the type locality in Chihuahua in the Sierra Madre Occidental. Notes: The new species is named in honor of Drs. C. F. and W. L. Culberson who have contributed considerably to our knowledge of the Parmeliaceae in general and to the Mexican flora in particular. Morphologically this new species resembles Hypotrachyna bogotenis (Nyl.) Hale in its habitat, in having broad, sublinear to subirregular lobes with dense, dorsiventral lobulae. However, whereas H. bogotensis is coriaceous, gray in color (atranorin and chloroatranorin in the upper cortex) and contains lecanoric acid in the medulla, H. culbersoniorum is very fragile, gray-green in color (isousnic acid in the upper cortex) and contains echinocarpic and conechinocarpic acids in the medulla. Hypotrachyna culbersoniorum is only the second species of Hypotrachyna known to contain isousnic acid in the upper cortex.