The defense toxins, spirostomins A and B, have been isolated as a diastereomeric mixture from the ciliate microorganism Spirostomum teres. The structure of spirostomin was elucidated through a number of NMR experiments which allowed assigning the unprecedented spiro[(2,5-dimethyl-5,6,7,8-tetrahydronaphthalene-1,4-dione)-8,6'-(pyrane-2',5'-di-one)] skeleton to this natural compound. The total syntheses of the racemic spirostomins confirmed their structure and relative configurations.