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The gamma-butyrolactone acid (1) and two known compounds, (-)-isomuronic acid and the tridepside gyrophoric acid, have been isolated from the lichen Punctelia microsticta. The structure and stereochemistry of compound 1 were determined on the basis of spectroscopic evidence and molecular modeling. Spectroscopic and physical data of 1 and (-)-dihydropertusaric acid, previously isolated from the lichen Pertusaria albescens, showed that both are the same compound, although for the latter the epimeric structure 2 has been proposed.