Bathelium is a genus of lichen-forming fungi in the family Trypetheliaceae.
The genus was circumscribed in 1803 by Swedish lichenologist Erik Acharius, with Bathelium mastoideum assigned as the type species.
Genus Bathelium features a thallus (a protective outer layer) usually with olive-green to brownish or greyish hues. This genus is distinguished by the form of its ascomata, the spore-producing structures, which possess apical ostioles (openings at the top). These ascomata can be found either singly or in groups, and often form, a stroma made of both thallus tissue and bits of host tissue. They are noticeable, ranging from to, and show a brown-black colouration, though in rare instances, they may have a whitish (powdery) appearance. This is complemented by a peripheral layer made up of tightly packed cells.
Within the ascomata, the, a tissue layer containing filamentous structures, is clear and hyaline (translucent). It comprises thin, interconnecting filaments known as . The number from one to eight per ascus. They are transversely septate to (divided into compartments by cross walls), with thin septa and more or less angular . These spores are, meaning they have a spindle-like shape, with either acute (pointed) or rounded ends, and are not constricted at the median septum. They appear hyaline, do not react to iodine (IKI−), and are encased in a gelatinous sheath.
No conidiomata (structures responsible for asexual reproduction) have been identified in this genus. In terms of chemical properties, Bathelium frequently contains anthraquinones, compounds often present internally in pseudostromata. Lichexanthone, a fluorescent xanthone compound, is occasionally found within the genus.