Two new labdane-class diterpenes, cacofurans A (1) and B (2), have been isolated from a sponge Cacospongia sp. Their structures were determined by analyzing spectroscopic data, by chemical transformations, and by X-ray diffraction. Cacofurans 1 and 2 inhibited the development of fertilized sea urchin eggs at concentrations of 0.5 and 5 microg/mL and showed an actin-disrupting effect on the NBT-II cell line at 10 microg/mL.