Two new norsesterterpenes, rhopaloic acids B (2) and C (3), have been isolated from the marinesponge Rhopaloeides sp. together with the known rhopaloic acid A (1). Their structures have been elucidated by spectroscopic methods. Compounds1, 2, and3 potently inhibited gastrulation of the starfish (Asterina pectinifera) embryo, whereas hydrogenation or esterification resulted in loss of the inhibitory