Thallus: foliose, adnate, 2-6 cm in diam., irregularly lobate lobes: subirregular, elongate, plane to subconvex, separate and contiguous to somewhat imbricate, 1-3 mm wide, not lobulate; apices: subrotund, smooth to crenate, eciliate upper surface: yellow-green, smooth, shiny, epruinose and emaculate isidia: moderate to abundant, initially subglobose, soon becoming cylindrical to irregularly inflated, 0.1-0.2 mm in diam., 0.2-1.5 mm tall; tips: syncorticate, pale brown, sometimes erumpent; soralia and pustulae absent medulla: white, with continuous algal layer lower surface: pale tan or brown, plane, densely rhizinate; rhizines: pale brown, simple to sparingly branched, 0.2-0.4 mm long Apothecia: not observed Pycnidia: rare, immersed conidia: bifusiform, 5-7 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with usnic acid (major); medulla with diffractaic acid (major), barbatic and squamatic acids (both minor), baeomycesic and 3-α-hydroxybarbatic acids (both trace) and sometimes 4-O-demethylbarbatic, 2-O-methylobtusatic, constipatic and protoconstipatic acids (all minor or trace). Substrate and ecology: on acidic rocks, often in open, arid habitats World distribution: southwestern North America Sonoran distribution: occasional at lower elevations in Arizona, Baja California Sur, Sinaloa and Sonora. Notes: Xanthoparmelia ajoensis is a relatively narrow-lobed, isidiate species with a pale brown lower cortex and has diffractaic acid as the major medulary secondary compound together with associated metabolites.