The ability of Neurospora crassa FGSC 4335 in the biotransformation of hydrocortisone was investigated. The microorganism produced two major metabolites after incubation with the substrate for seven days. Each microbial product was purified chromatographically and identified on the basis of spectral data. The products were identified as 11?,17?,20?,21-tetrahydroxypregn-4-en-3-one (II) and 11?-hydroxyandrost-4-en-3,17-dione (III) in the yields of 25% and 18.8%, respectively. Time course study showed the accumulation of both products from the second day of the fermentation and reached to the maximum in the seventh day. Optimum concentration of the substrate, which gave maximum bioconversion efficiency, was 1 mg ml?1 in one batch. Biotransformation was completely inhibited in a concentration above 2.5 mg ml?1.