A new tetramic acid Mg salt, ancorinoside A Mg salt, has been isolated from the marine sponge Ancorina sp. together with the known ancorinoside A. The structure of ancorinoside A Mg salt has been established on the basis of spectroscopic and chemical investigations and chemical correlations. Ancorinoside A Mg salt potently inhibited blastulation of the starfish (Asterina pectinifera) embryo.