A novel norsesterterpene, rhopaloic acid A, was isolated from a marinesponge, Rhopaloeides sp. The structure was determined to be 2-methylene-6-(4,8,12-trimethyl-3,7,11-tridecatrienyl)-3,7-epoxyheptanoic acid on the basis of its spectroscopic data. Rhopaloic acid A inhibited gastrulation of starfish embryos and also exhibited potent cytotoxicities in vitro against human myeloid K-562 cells, human