5?, 8?-Epidioxy-24(S)-ethylcholest-6-en-3?-ol (1) was isolated from the marine sponge Biemna triraphis Topsent collected at Andavadoaka, near Toliary (west coast of Madagascar-Indian Ocean). The structure was elucidated from NMR spectroscopic data. This is the first report of this compound in a marine sponge belonging to the Biemna genus.