Clitopilus passeckerianus is a white-rot agaric fungus in the family Entolomataceae notable for its role as a natural producer of pleuromutilin, a diterpenoid antibiotic compound that serves as the biosynthetic precursor for the veterinary and human antibiotic drugs tiamulin and valnemulin, used to treat Mycoplasma infections in livestock. First described from European woodland habitats, the species produces the characteristic Clitopilus fruiting body: a pale, whitish cap with decurrent, crowded gills becoming pinkish at maturity from angular spores, and a farinaceous odor. Its antibiotic-producing capacity makes C. passeckerianus of significant pharmaceutical interest, and biosynthetic studies of pleuromutilin production have informed synthetic chemistry approaches to antibiotic development. The species inhabits temperate deciduous woodland floors in Europe, fruiting in autumn among leaf litter. Discovery of its pleuromutilin biosynthetic pathway has opened avenues for heterologous expression and semi-synthetic modification aimed at developing novel antibiotics to counter antibiotic-resistant bacterial pathogens.