Glycyrrhizin (GL), the well-known sweet saponin of licorice, has been used as a food-additive and as a medicine. Its aglycone, glycyrrhetic acid (GA) showed antiinflamatory, antiulcer and antiviral properties. GA is now produced form GL by acid hydrolysis. However, it is difficult to obtain GA in a good yield by using this method, because many by-products are also produced. Screening of different microorganisms (13 bacteria, 2 yeasts and 23 fungi) for production of GA from GL revealed that Aspergillus niger NRRL 595 produced the highest yield of GA. The bioconversion of GL by A. niger NRRL 595 for 96 h, followed by isolation and purification of the transformation products led to the separation of two conversion products, namely: GA and 3-oxo-GA. Confirmation of the identity of these products was established by determination of their Rf values, m.p., and IR, UV, MS and NMR spectra. The conditions for cultivation of this fungus with the maximum hydrolytic activity for the maximum yield of GA were investigated. Based on the results, A. niger NRRL 595 was cultivated with a medium composed of 1.75 % GL, 0.5 % glucose, 0.8 % corn steep liquor at pH 6.5 at 32 °C for 96 h. The cultivation of fungal cells under the latter conditions afforded GA and 3-oxo-GA in a yield of 65 % and 22 %, respectively.