Rich collections of foliicolous lichens were made, especially from submontane and montane areas, in the Usambara Mountains of Tanzania during the Usambara Rain Forest Research Project expeditions in 1982, 1984 and 1985. An enumeration of the identified species is provided together with taxonomic, ecological and geographical notes. This first part treats 36 species from the Arthoniaceae, Opegraphaceae and all genera with pyrenocarpous ascomata. New records include 14 species for the Usambaras, eight for Tanzania, and three to Africa. Porina sphaerocephaloides sp. nov., widely distributed in the study area, is described.