AbstractFifteen specimens of four Acarospora species of subgenus Xanthothallia were analysed. All species contained (+)-rhizocarpic acid. A. gobiensis and A. schleicheri had only this compound and A. oxytona this and (+)-lichesterinic acid. A. chlorophana seems to exist in two chemical races, one with a mixture of (-)-acaranoic and (-)-acarenoic acids and the other with (+)-roccellic acid. The stereochemistry and biogenesis of these compounds is briefly discussed.