Halicholactone (1) and neohalicholactone (2), unusual fatty acid metabolites having a cyclopropane ring and a nine-membered lactone were isolated from the marine sponge Halichondria okadai Kadota. The planar structures of the new metabolites were elucidated on the basis of spectral and chemical means. Marine sponges have yielded a variety of architecturally novel and pharmacologically interesting compounds. 1 We have examined the constituents of the sponge Halichondria okadai Kadota collected at Dai%aki, Mie Prefecture, Japan and isolated two new fatty acid metabolites, halicholactone (1) and neohalicholactone (2). We wish to report herein the structural elucidation of these metabolites on the basis of spectral data coupled with chemical evidence. The EtOAc-soluble material from the MeOH extract of the sponge was subjected to chromatography on silica gel (benzene-EtOAc + EtOAc -+MeOH) followed by reversed-phase HPLC [ODS, MeOH-H20 (7:3)] to give halicholactone (1)' (colorless oil; 2 x 10m3% dry weight) and neohalicholactone (2)3 [mp 69-70 'C (pentane-ether); 6 x 10W4% dry weight].