The lichenicolous genus Plectocarpon (Roccellaceae, Arthoniales) is mono? graphed. Thirty two species are accepted, of which fifteen are new to science ( Pl. bunodophori sp. nova on Bunodophoron patagonicum, Pl. concentricum sp. nova on Pseudocyphellaria homoeophylla, Pl. coppinsii sp. nova on Ps. crocata, Pl. cristalliferum sp. nova on Sticta sp., Pl. latisporum sp. nova on PS. coriifolia, Pl. melanohaleae sp. nova on Melanohalea ushuaiensis, Pl. obtectum Sp. nova on Ps. encoensis, Pl. Opegraphoideum sp. nova on Ps. homoeophylla and Ps. multgfida, Pl. pseudoleuckertii sp. nova on Ps. coriifolia, Pl. serusiauxii sp. nova on Sticta sp., Pl. sticticola sp. nova on Sticta spp., Pl. tibellii sp. nova on P s. rubella, Pi. triebeliae sp. nova on Lobaria quercizans, Pl. venustum sp. nova on Nephroma papillosum, and Pi. violaceum Sp. nova on Nephroma antarcticum ), two are newly combined in the genus ( Pl. gallowayi comb. nova, bas. Melaspilea gallowayi, and Pi. leuckertii comb. nova, bas. Opegrapha leuckertii ), and one is left unnamed ( Pl. sp. on Usnea ). Seven species examined during the study are included in other genera: Arthonia sampaianae comb. nova (bas. Pl. sampaianae ), Arthonia sp. (on Sticta leami ), Enterographa epiphylla comb. nova (bas. Chiodecton epiphyllum ), E. punctata sp. nova (on Lobaria sp.), Opegrapha phaeophysciae sp. nova (on Phaeophyscia hispidula ), Perigrapha nitida sp. nova (on Ps. glabra ), and Sigridea labyrinthica comb. nova (bas. Plectocarpon labyrinthicum ). Lectotypes are designated for Celidium dubium and Lichenomyces parasiticus. A key is given to all known genera of Roccellaceae containing lichenicolous species, and another key to the species of Plectocarpon. Species of Plectocarpon are especially diverse on Lobariaceae and Nephromataceae: Pseudocyphellaria (at least 11 species), Lobaria (6 species), Sticta (3 species), Nephroma (3 species) and Lobarina (1 species), and most of them are confined to one or a few closely related host species. Regions with a high species diversity are Australia and New Zealand (7 species), southern South America (10 species), and the boreal region (8 species). Host-specificity of Pl. lambinonii and Pi. lichenum, together with morphological and molecular data suggest that Pseudocyphellaria anomala and Ps. anthraspis might belong to Lobaria s. str.