We present a survey of the six known species of Arthothelium occurring in the western Mediterranean. The oceanic A. lirellans, which may occur in the study area, is included for comparison; A. taediosoides is described as new and a key to all species is given. Arthothelium as understood today is composed of more-or-less distinct natural groups of species, some of them being compared here. Representatives of the A. sardoum group are related to species of the Arthonia radiata or A. galactites group on the basis of anatomical characters of the ascomata.