Life habit: lichenicolous Perithecia: globose to pyriform, immersed or sessile, (100-)150-250(-400) µm in diam. wall: brown to black, mainly 3-8 cell layers thick, upper part often thickened, lower part brown or red brown periphyses: present, (1-)2-3 µm in diam., up to 40 µm long asci: broadly clavate to subcylindrical, (30-)40-65 (-80) x (11-)15-23 µm, 16-64-spored ascospores: pale-brown to dark brown, 1-septate, ovoid to ellipsoid, 610(-13) x 3-5.5(-6) µm, partly slightly aciculate, smooth or slightly warted, thin- or thick-walled Pycnidia: immersed, 45-90(-100) µm in diam. conidiogenous cells: (4-)5-7(-8) x (2-)3-4(-5) µm conidia: hyaline, (2.5-)35(-5.5) x (0.5-)1-1.5(-2) µm. Hosts: thalli and more rarely in apothecia of more than 100 species of mainly crustose, often saxicolous, lecanoralean lichens World distribution: world-wide in extratropical regions Sonoran distribution: relatively common throughout Arizona and southern California. Notes: In the Sonoran region Muellerella pygmaea is known on Acarospora, Lecidea, Lecidella, Protoblastenia and Rhizocarpon. At the moment three entities are distinguished at the variety level, of which var. athallina is the most common. pygmaea: occurs mainly on Lecidea s. str., but also on Bellemerea and other saxicolous crustose lichens at higher altitudes. It is characterized by slightly warted, thick-walled, medium to dark brown ascospores of (7.5-)8-10(13) x (3.5-)4-5(-6) µm and 20-32-spored asci. Finally, var. ventosicola: (Mudd) Triebel occurs on Ophioparma (host of the type specimen) and Rhizocarpon at higher altitudes. It has broadly ovoid, thick-walled, smooth ascospores of 6.5-8.5(-9) x 4-5.5(-6) µm. The asci are 32(-64)-spored.