We describe Acarospora carrizoensis (Acarosporaceae) as a new species of saxicolous lichen from Carrizo Plain National Monument in central California. The total number of taxa in the family Acarosporaceae documented during preliminary lichen floristic surveys of the Monument is now at 22 species, with at least one more new species to be described. Keywords: Acarosporaceae, Acarospora americana, biodiversity, Southwestern North America.