A novel tridecadepsipeptide, theonellamine B, has been isolated from an Okinawan marine sponge of the genus as a specific inhibitor against Na,K- ATPase. Its structure has been determined to be a lactone ofmethoxyacetyl--Val-- MeLeu--Thr-β-Ala--Leu--MeIle-β-Ala---Ile--MeVal--MeAla-β-Ala--Leu--- MeIle on the basis of chemical reaction and spectral data including two dimensional homo- and heteronuclear