Basidiomata: rarely resupinate over the thallus, generally inducing the formation of distinct galls on the host thallus, frequently with a strongly constricted base, often lobate, sometimes tuberculate, margin sometimes ciliate, lower surface of lobes: concolorous with the lichen thallus or pale brown; upper surface: pale brown, reddish brown, rarely orange or dark brown, smooth, shiny, rarely dull or pruinose, 0.4-4 mm in diam. hyphae: context hyphae: 1.5-3 µm in diam.; clamps: not observed haustorial branches: not observed hymenium: containing numerous probasidia and conidiogenous cells, intermixed with brown hyphae of the host; probasidial initials: clavate, ellipsoid or subspherical; clamps: not observed; hyphidia and cystidia: absent basidia: when mature, ellipsoid to clavate, often with an attenuated base, with 1-3 longitudinal, oblique or transverse septa, 12-23(-33) x 5-15 µm epibasidia: 2-3-5 µm in diam., up to 40 µm long basidiospores: ellipsoid, with a distinct apiculus, 5-8 x 4-7 µm. Hosts: thalli of Parmotrema and Rimelia species, usually inducing the formation of large, conspicuous galls World distribution: cosmopolitan Sonoran distribution: Baja California Sur and southern California. Notes: Tremella parmeliarum is a relatively common taxon with large, convex, brown basidiomatal galls. It is apparently confined to Parmotrema and Rimelia species.