Thallus: crustose, leprose, without lobes, diffuse, continuous or convergent, forming thalline colonies up to 10 cm wide composed of interconnected granules, stratified with distinct medulla and hypothallus, up to 0.7 mm thick upper surface: white- to gray-blue (in Sonoran specimens) to yellowish white, uneven, often eroding, exposing the underlying medulla and hypothallus soredia: granules, adhering to one another, usually spherical, without projecting hyphae, up to 80 um in diam., subcorticate with hyaline paraplectenchyma, mostly 5 µm in diam. or smaller, surrounding an algal core medulla: white and cottony, 0.3-0.5 mm thick, composed of mostly gelatinized hyphae, indistinct to irregularly paraplectenchymatous to prosoplectenchymatous prothallus: composed of hyaline, distinct prosoplectenchyma, 3-5 µm wide, forming a loose entangled layer lower surface: pale brown, irregular and discontinuous, apparently composed of necral, gelatinized material, adhering to substrate Spot tests: K± yellow (atranorin), KC-, C-, P± yellow (atranorin) Secondary metabolites: atranorin and roccellic and/or angardianic acid (major), and (in one Sonoran specimen) gyrophoric acid (minor) and ursolic acid (minor). Substrate and ecology: on granite and other acid rocks, overgrowing mosses and other lichens, at 1000-1500 m World distribution: Antarctica, Europe, Greenland, and North America Sonoran distribution: southern California (San Jacinto Mountains, Riverside Co.; Palomar Mountain and Warner Hot Spring, San Diego Co.) Notes: Lepraria alpina is similar to L. jackii Tønsberg and L. atlantica Orange in chemistry, but L. alpina is morphologically distinct with a hypothallus of entangled hyphae and a different ecology, preferring acidic rocks in open habitats, undisturbed by snow cover. Lepraria jackii occurs on bark and L. atlantica grows on shaded rock in coastal regions of Great Britain, Ireland and Scandinavia. Porphyrilic acid was not observed in Sonoran specimens. Our concept of L. alpina is based on morphology of the hypothallus and ecology, rather than the occurrence of porphyrilic acid. Thus far, in our study of Lepraria in southern California chemotype 3 of Lepraria caseioalba is considered to be conspecific with L. alpina.