Arthonia frostiicola and A. galligena are described as new to science based on collections from mountainous regions of southeastern North America. Arthonia frostiicola infects the saxicolous lichen Dirinaria frostii, producing emarginate black apothecia which erupt from within the host thallus. It is characterized by a dark hypothecium and 1-septate, obovoid ascospores which turn brownish and verruculose in age. It is known from five collections made in the southern Appalachian Mountains and Ozark Mountains in southeastern North America. Arthonia galligena produces galls in the thallus and apothecia of the corticolous lichens Lecanora masana and L. rugosella, and is apparently endemic to the high elevations of the southern Appalachian Mountains. It is characterized by a variably pigmented, pale to red-brown hypothecium and 2-septate, macrocephalic ascospores which turn brownish and verruculose in age. Keys to the species of Arthonia on Caliciales and Lecanoraceae are provided. Keywords: New taxa, biodiversity, endemism, host-parasite relationships, lichenicolous fungi, taxonomy.