Two novel nucleosides, mycalisines A and B, have been isolated from a marine sponge and their structures elucidated as 4-amino-5-cyano-7-(3-0-methyl-5-deoxy-a-D-erythro-pent-4 enofuranosyl)-pyrrolo[2,3_d]pyrimidine and 5-cyano-7-(3-V-methyl-5-deoxy-B-D-erythro-pent-4 enofuranosyl)-pyrrolo[2,3-dlpyrimidine-4-one, respectively. Both compounds inhibit cell division of fertilized starfish eggs.