Akaeolide, a novel polycyclic polyketide, was isolated from the culture extract of a marine-derived actinomycete belonging to the genus Streptomyces. The planar structure of the new compound was elucidated by spectroscopic analysis including NMR and MS, and the absolute configuration was determined by X-ray crystallographic analysis of its chlorinated derivative. Akaeolide possesses a 15-membered carbocyclic framework, apparently derived from the malonate pathway, with a tetrahydrofuran ring and a β-keto-δ-lactone unit.