Thallus: areolate, rarely partly rimose, 2-8 cm in diam., (0.1-)0.2-0.5(-1.2) mm thick areoles: angular, sometimes verrucose or irregular, flat to convex (especially in the middle of the thallus), rarely aggregated and uneven, (0.2-)0.4-1.0(-1.8) mm in diam., contiguous, separated by distinct cracks, rarely dispersed prothallus: sometimes present at the thallus edge, thin, continuous or fimbriate, black to blue-black or brown-black, 0.1-0.5(-1.3) mm wide surface: brown to gray-brown, sometimes gray, rather often showing a mosaic pattern on the areoles, rarely olive-brown or ochre, often with white to gray spots, lines and rims; dull to ±shiny upper cortex: (10-)20-35(-50) µm thick, uppermost part ±brown, 5-12(-18) µm thick, with cells (3-)5-7(-9) µm in diam.; cortex covered with an epinecral layer 1-10(-20) µm thick photobiont: chlorococcoid, cells ±round, 5-17 µm in diam. Apothecia: aspicilioid, usually rather common, (0.1-)0.2-0.5(-1) mm in diam., 1-2(-5) per areole, round to angular or sometimes elongated disc: black, usually concave, rarely plane, without pruina or sometimes with ±white pruina thalline margin: flat to ±elevated, sometimes prominent, often with a white to gray rim, or concolorous with thallus, rarely darker exciple: (15-)30-60(-70) µm wide, I- or partly I+ blue medially, rarely entirely I+ blue; uppermost cells brown to olive-brown, ±globose, (4-)5-6(-7) µm in diam. epihymenium: green to olive or olive-brown, rarely brown, without or with a few crystals, rarely with numerous crystals, N+ green to blue-green, K+ brown hymenium: hyaline, I+ persistently blue or sometimes partly turning yellow-green, (120-)140-200(-240) µm tall paraphyses: submoniliform to moniliform, rarely non-moniliform, with (0-)1-5(-8) upper cells ±globose to sometimes subcylindrical, 3-4.5(-5) µm wide, in lower part (1-)1.5-2(-2.5) µm wide, slightly branched and anastomosing subhymenium and hypothecium: pale, I+ persistently blue, together 40-70 µm thick asci: clavate, (70-)80-130 x (17-)20-30(-34) µm, 8-spored ascospores: hyaline, simple, ellipsoid, (17-)18-28(-32) x (9-)10-16(-20) µm Pycnidia: rare to rather common, 1-2(-4) per areole, immersed, often with a white rim, rarely aggregated (up to 600 µm in diam.), (110-)130-220 µm in diam., with a black, punctiform to sometimes elongated ostiole, (50-)60-120(-160) µm in diam. conidia: filiform, straight or slightly curved, (7-)9-13(-15) x (0.6-)0.8-1(-1.2) µm Spot tests: cortex and medulla I-, K-, P-, C- Secondary metabolites: none detected by TLC, or rarely an unknown substance (Rf 0-1 and brown-black in TLC C-system) found in 4 specimens, including the type. Substrate and ecology: on exposed to somewhat shaded, siliceous rock, rarely on serpentine World and Sonoran distribution: southern and central California, at 120-1770 m. Notes: Aspicilia phaea has usually a brown to gray-brown thallus color and the apothecia are often surrounded by a gray to white thalline rim. Furthermore it has a tall hymenium, large spores (usually over 20 µm), filiform and rather short conidia, and it lacks secondary substances or rarely has an unknown substance. DNA studies (ITS) on two specimens, including the type (Owe-Larsson 9126 & 9158) shows that A. phaea is related to A. confusa and A. glaucopsina, which both have longer conidia. Aspicilia guadalupensis is microscopically quite similar to A. phaea, but differs by the olive tinge of the thallus, the smaller apothecia, and by the well developed prothallus with a white outer margin. Aspicilia santamonicae also shows similarities to A. phaea, but differs by having norstictic acid, while A. nashii differs by having longer conidia and a different shade of brown thallus color. A few specimens from Arizona with rather short conidia agree quite well with A. phaea but need further study.