Studies on the microbialtransformation of the sesquiterpene endoperoxide artemisitene have revealed that artemisitene was metabolized by Aspergillus niger (NRRL 599) to yield 11-epi-artemisinin, 9 beta-hydroxydeoxy-11-epi-artemisinin and 9 beta-hydroxy-11-epi-artemisinnin. These metabolites were characterized on the basis of their spectral data.