The lichen Peltula euploca containing coccal cyanobacteria grows in arid and semi-arid localities in the Mediterranean. The lichen Glyphopeltis ligustica containing green algae often develops on the thalli of Peltula. Both lichens show different methods of reproduction and dispersal which, in connection with their type of growth and their strategies of colonization, are effective adaptations to the extreme conditions of the habitat. Principal aspects of mutualism and parasitism are discussed.