Biotransformation of artemisinin (1) by Aspergillus niger was investigated. During 12 days at 28 A degrees C and pH 6.0, A. niger transformed artemisinin into four products. They were identified as 3 beta-hydroxy-4,12-epoxy-1-deoxyartemisinin (2), artemisinin G (3), 3,13-epoxyartemisinin (4), and 4 alpha-hydroxy-1-deoxyartemisinin (5). Products 2 and 4 are new compounds and are being reported here for the first time. The product 4 contains a 3,13-epoxy structure. This is the first report of epoxidation of artemisinin using microbial strains. The product 4 still has an intact peroxide bridge and therefore can be used as a scaffold for further structural modification using chemical and biological methods in the search for new antimalarial drugs.