Mycelium: immersed, invisible Pseudothecia: black, sessile, spherical at first, apothecioid when mature, 0.2 hymenium: hyaline, pale brown in the uppermost part, 70-80 µm tall, I-; paraphysoids: branched and with anastomoses, 2-3 µm thick, with slightly enlarged end cells, embedded in a pale brown gel asci: broadly cylindrical, 50-60 x 12-17 µm, mostly 8-spored, rarely single asci with only 6 or 4 ascospores ascospores: remaining hyaline for a long time, finally pale brown, 1-septate, (12-)13-17 x 6-8.5 µm Hosts: commensalic to slightly parasitic on thalli of Phaeophyscia cernohorskyi, P. hirsuta, C. hispidula and P. orbicularis; outside the Sonoran region also on P. sciastra (host of the type), P. endococcinodes and P. cf. hirtuosa; host thalli not or only slightly discolored by the infection World distribution: known from several scattered localities in Europe, Asia (Japan), Africa (Canary Islands), North America and South America (Peru) Sonoran distribution: known from several localities in California with an altitudinal range from 300 to 2300 m. Notes: Although Phaeophyscia species are among the most common and widely distributed lichens, the majority of the populations are not infested with Buelliella physciicola. The species seems to be +restricted to undisturbed dry habitats with host populations growing there for a long time. Thus, Buelliella physciicola is normally found growing on thalli of populations of Phaeophyscia species on rocks living under semi-arid climatic conditions. Thalli of Phaeophyscia species on rock and soil may also be infested with the endoparasitic lichen Buellia pulverulenta (Anzi) Jatta, but this species is easily distinguished by its 3-septate ascospores and the lecanoralean asci.