Microbial transformation of a potent progestin, norethisterone (17β-hydroxy-19-nor-17α-pregn-4-en-20-yn-3-one) (1) was carried out by using two filamentous fungi Aspergillus niger and Penicillium citrinum.Biotransformation of 1 with A. niger yielded a hydroxylated transformed product 10β,17β-diydroxy-19-nor-17α-pregn-4-en-20-yn-3-one (2) whereas 11β,17β-diydroxy-19-nor-17α-pregn-4-en-20-yn-3-one (3) was obtained through microbial transformation of 1 by P. citrinum. It is the first report of their production from 1 by using A.niger andP. citrinum with complete 1 H- and 13 C-NMR assignment. The structures of both metabolites were characterized by various spectroscopic techniques and reported data.