Acarospora is a poorly known crustose lichen genus commonly found in the southwestern United States and Mexico. The genus is commonly divided into two subgenera: A. subgen. Xanthothallia and A. subgen. Phaeothallia (or A. subgen. Acarospora). These subgenera are generally distinguished by chemistry. Members of the subgen. Xanthothallia contain the intense yellow pigment rhizocarpic acid, thus exhibiting a yellow color, whereas members of the subgenus Phaeothallia lack rhizocarpic acid and are usually brown. The use of secondary products as taxonomic characters in Lichenology has gained popularity over the last 30 years. As the initial stage of a thorough study of this subgenus in the greater Sonoran Desert, the secondary products of over 800 specimens were analyzed using thin layer chromatography (TLC). One new secondary product was found (xanthothallic acid), and four secondary products were recognized for the first time in this subgenus (ovoic acid, 4-0-demethylnotatic acid, hyproprotocetraric acid, and eumitrin A1).

Key words: Acarospora, chemistry, xanthothallic acid