Five new aromatic polyketides, alternaphenols A–E (1–5), and two known compounds (6–7) were isolated from a liquid culture of Alternaria sp. (strain no. NF2198), a caterpillar associated fungus. Their structures were determined by extensive spectroscopic analysis and single crystal X-ray crystallography. Compounds 1–7 were evaluated for their cytotoxic activities against a human skin melanoma A-375 cell line. Only compounds 4 and 5 showed weak cytotoxic activities.