Astragaloside IV (ASI) is the main active constituent of astragalosides in Radix Astragali. Its content in natural plants is relatively low. We selected the strain Absidia corymbifera AS2 to transform astragalosides to ASI. This strain enhanced ASI production approximately fourfold when cultures were supplemented with 5 g/L of crude astragalosides. ASI was purified from the fermentation culture by a macroporous resin with a chromatographic purity of 97.2% and recovery of 63.7%. Astragaloside I, isoastragaloside I, astragaloside II and isoastragaloside II were identified as the biotransformation precursors of ASI, and their biotransformation time course profiles differed. To our knowledge, this is the first report on the production of ASI by microbial transformation and the investigation on its biotransformation precursors. (C) 2011 Elsevier Ltd. All rights reserved.