Five new (1, 2, 4-6) and one known (3) diterpenoid were isolated from the keratose sponge Spongia (Heterofibria) sp. Structures of these compounds and their absolute configurations were proposed on the basis of X-ray analysis of 1, its CD spectrum, and NMR and MS spectroscopic studies of 1-6. One of the new diterpenoids was shown to be 2(R),3(S),4(S)-3,18-methylene-2-acetoxyspongia-13(16),14-diene (6), possessing a novel carbon skeleton system.