The structure and absolute configuration of the novel tetracyclic diterpenoid aphidicolin, an antimitotic and antiviral metabolite of Cephalosporium aphidicola Petch, have been determined, and a systematic nomenclature is proposed. Possible biosynthetic routes to aphidicolin are discussed. Derivatives of aphidicolin, including possible biogenetic precursors and the C(3)- and C(16)-epimers of the antibiotic have been prepared. The high resolution i.r. hydroxyfrequencies are reported for a number of ring-A 1,3-diols derived from aphidicolin.