An unusual nucleoside, 4-amino-7-(5'-deoxy-beta-D-xylofuranosyl)-5-iodopyrrolo[2,3-d]pyrimidine (1), was isolated from an ascidian, Diplosoma sp., and its structure was successfully determined by spectroscopic and chemical analysis. The known didemnenones 2-5 were also isolated. Compound I was found to inhibit the division of fertilized sea urchin eggs.