As shown by quantitative (1)H NMR measurements, a lipophilic extract of the aerial parts of Hypericum atomarium ssp. degenii contained a high percentage (3.1% per weight of dried plant material) of a prenylated phloroglucinol (1). Compound 1, named hyperatomarin, occurring in two tautomeric forms (1a <==> 1b), was isolated by bioactivity-guided preparative TLC and was identified on the basis of spectral data interpretation. This isolated phloroglucinol exhibited activity against Gram-positive (Staphyloccocus aureus and Microccocus luteus) and Gram-positive spore-forming bacteria (Bacillus subtilis and B. IP 5832).