A new [11]cytochalasin derivative, xylarisin (1), was isolated from the marine-derived fungus Xylaria sp. PSU-F100 along with six known metabolites: three mellein derivatives (2-4), one pyrone derivative (5) and two carboxylic acids (6,7). The structure and stereochemistry of 1 were determined by NMR and X-ray diffraction analyses. All isolated compounds showed mild antibacterial activity against standard Staphylococcus aureus ATCC 25923 and methicillin-resistant strain.