Bioassay-guided fractionation of an extract of the lichen Cladonia incrassata against Staphylococcus aureus led to a novel compound, 1,5-dihydroxy-2,4,6-trichloro-7-methylxanthone (1), along with six known compounds: (−)-usnic acid (2), didymic acid (3), condidymic acid (4), squamatic acid (5), thamnolic acid (6), and prasinic acid (7). Didymic, condidymic, and prasinic acids were isolated for the first time from C. incrassata. Didymic, condidymic, and (−)-usnic acids were active against S. aureus (a minimum inhibitory concentration of 7.5 μg/mL). Key words: xanthone; Cladonia incrassata; Cladoniaceae; antibacterial; Staphylococcus aureus; structure elucidation.