The species of Arthonia s. lat. (Arthoniales, Ascomycotina) lichenicolous on species of Pseudocyphellaria and other Lobariaceae are revised. Thirteen species are accepted, and eight species are described as new (all from the Southern Hemisphere): Arthonia badia Wedin & Hafeliner, A. coriifoliae Wedin & Hafeilner, A. fiavicantis Wedin & Hafellner, A. maculiformis Wedin & Hafeliner, A. minuta Wedin & Hateliner, A. santessoniana Wedin & Hafellner, ‘A.’ semi-immersa Wedin & Hafeilner, and A. subaggregata Wedin & Hafellner. Comparative notes on additional accepted species previously described from Pseudocyphellaria or other Lobariaceae (A. pelvetii, A. plectocarpozdes, A. pseudocyphellariae, A. stzctaria, and A. subconveniens) are included, and a key to the Arthonia (and similar) species growing on Lobariaceae is presented. The coelomycete genus Subhysteropycnis Wedin & Hafellner is described for the species S. maculiformans Wedin & Hafellner, the macroconidal anamorph of Arthonia badia. The lecanoralean genus Corticiruptor Wedin & Hafeliner is described as new to accommodate the single lichenicolous species C. abeloneae (P. M. Jørg.) Wedin & Hafellner comb. nov., and the additional new combination Plectocarpon linitae (R. Sant.) Wedin & Hafellner is proposed. The names Celidium pelvetii Hepp, Sticta aurata b abortiva Schaer. and Arthonia stictarza Nyl. are lectotypified, and the typifications are discussed.