[3] Usnic acid was identified in many genera of lichens including Usnea, Cladonia, Hypotrachyna, Lecanora, Ramalina, Evernia, Parmelia and Alectoria.

Although it is generally believed that usnic acid is exclusively restricted to lichens, in a few unconfirmed isolated cases the compound was found in kombucha tea and non-lichenized ascomycetes.

It is believed that usnic acid protects the lichen from adverse effects of sunlight exposure and deters grazing animals with its bitter taste.

[13] It is possible to determine the content of usnic acid in lichen extract using reversed-polarity capillary zone electrophoresis or high performance liquid chromatography analysis.

[14] A spot test using anisaldehyde reagent was developed to detect usnic acid in lichens, producing a distinctive magenta color reaction that works reliably in the presence of most other lichen substances and can aid in identification, particularly in species where visual detection of usnic acid's natural pale yellow color is difficult.