Sarcogyne similis is found to produce psoromic acid, a substance previously known from only one other species in the family Acarosporaceae. Study of more than one hundred specimens from throughout the range of S. similis confirms the species is widely distributed from Nova Scotia, Canada, south throughout much of the eastern United States, and westward through the Sonoran Desert to southern California, where it occurs on non-calcareous rocks, especially sandstone. The recently proposed synonymy with S. reebiae is confirmed. Sarcogyne similis f. convexa is lectotypified with material that contains psoromic acid. The type of S. californica, currently treated as a synonym of S. similis, does not produce psoromic acid and may belong to a separate taxon. The distribution of S. dakotensis, which produces norstictic acid, appears to be nearly allopatric with S. similis. Keywords: Biogeography, endemism, nomenclature, para-phenylenediamine, spot tests, typification..