-B (4a) were isolated from the Okinawanmarinespongesp. along with known compounds, furospinulosin-1 (1) and ilimaquinone (2). The structures of dictyoceratin-A (3a) and -B (4a) were determined on the basis of spectral data and their absolute configurations were deduced to be antipodal to that of ilimaquinone (2) by comparing an ozonolysia product(5) with that (6) derived from ilimaquinone (2).