Thallus: foliose, closely adnate, frequently tightly wrapped around small twigs or tightly appressed to larger stems, (2-) 4-6 (-8) cm across, lobate lobes: 2-3 (-5) mm wide, ± linear-elongate, apically rounded; margins aprèssed; cilia absent upper surface: light yellowish green (often rather more yellowish than green, occasionally becoming dark olive green), strongly and finely wrinkled except near the margins, to more often becoming folded; without pseudocyphellae upper cortex: thin walled medulla: white lower cortex: thin walled lower surface: white to buff or partly olivaceous, reticulately wrinkled, rhizines sparse to more often abundant to the lobe tips, concolorous with or slightly darker than lower surface, usually simple; Apothecia: numerous, laminal, mostly restricted to thallus center, 1-4 mm diam.; disc: light brown to greenish or yellowish or darkening; exciple: 2-layered, upper layer composed of horizontally arranged hyphae, lower layer thin walled asci: rather broadly clavate, 40-50 x 13-18 µm; axial body 3-5 µm ascospores: spherical, 4-6 µm diam. Pycnidia: laminal, generally conspicuous, often abundant, black conidia: formed pleurogenously, bifusiform, 5-7 (-9) x c. 1 µm Spot tests: upper cortex K-, C-, KC+ yellow, P-, UV-; medulla K-, P-, KC-, P- Secondary metabolites: upper cortex with usnic acid; medulla with caperatic acid (major). Substrate and ecology: on conifers in montane to subalpine forests, 1500-2400 m in its southern range, rarely on hardwoods (Quercus) World distribution: common NW North America, particularly near the Pacific coast, but rarer to the south Sonoran distribution: formerly known from the southern California mountains and one location in Santa Cruz County in SE Arizona. Notes: The one occurrence in Arizona is a disjunct from its other extant populations, and occurs on an unusual bark type. In the Sonoran region Ahtiana sphaerosporella might be confused with the more southerly Flavoparmelia rutidota, from which A. sphaerosporella can be easily distinguished by its mostly pale discs and its P- medulla (lacking protocetraric acid). Flavopunctelia species are also yellow-green and are much more common in the Sonoran region, but they have white pseudocyphellae on the upper surface (not in A. sphaerosporella) and a C+ red medulla.