The microbial transformation of a cytotoxic bufadienolide, bufalin (1), was carried out using two strains of filamentous fungi. Cunninghamella blakesleana catalyzed the specific 12alpha-hydroxylation of bufalin and produced 12alpha-hydroxybufalin (2) and 7beta,12alpha-dihydroxybufalin (3) as the major metabolites, together with 7beta-hydroxybufalin (4) and 12beta-hydroxybufalin (5) in low yields. Two minor products were isolated from the culture broth of Mucor spinosus and were identified as 7beta,15alpha-dihydroxybufalin (6) and 5beta,7beta-dihydroxybufalin (7), respectively. Metabolites 2, 3, 6, and 7 are new compounds, and their structures were fully characterized by NMR and MS spectroscopy.