Biotransformation of ent-kaur-16-en-19-oic acid, the kaurene diterpenoid isolated from the plant Croton oblongifolius Roxb., was performed using Aspergillus niger. After 7 days of incubation, the culture was extracted with dichloromethane and the products were assayed by Thin Layer Chromatography (TLC). The TLC chromatogram showed three spots with Rf values of 0.68, 0.61 and 0.58. The dichloromethane extract was separated by column chromatography into three compounds. The structures of these three compounds were elucidated using spectroscopy technigue. Mass spectrum of compound 1(Rf 0.68) showed that the expected molecular weight is 316. Consequently, the expected molecular formula of compound 1 is C20H28O3. Compound 2(Rf 0.61) showed the expected molecular weight at 332 due to mass spectrometric data and the expected molecular formula of compound 2 is C20H28O4. The structure elucidation of compound 3(Rf 0.58) using mass spectrometry and X-ray crystallography showed that the molecular weight of compound 3 is 348 and the structure is ent-(7beta, 11alpha)-dihydroxy-1-oxo-kaur-16-en-19-oic acid
