Two novel steroidal 24-O-xylosides, designated as rathbuniosides R(1) (1) and R(2) (2), and the known amurensoside A (3) and 3-O-sulfomarthasterone (4) have been isolated from the starfish Asterias rathbuni. The structures of all the compounds were determined from their spectroscopic data, including one- and two-dimensional NMR methods. The compounds 1 and 4 inhibit the cell division of fertilized sea urchin eggs at doses of 7.0 x 10(-5) and 2.9 x 10(-5) M, respectively.