The microbial transformation of testosterone by the fungi Rhizopus stolonifer and Fusarium lini has been investigated for the first time. The bioconversion reactions observed from R. stolonifer included oxidation of a 17beta-hydroxyl group, a hydroxylation at equatorial 11alpha position, reduction of the double bond at C-4 with oxidation of the methylene at C-6 to the corresponding keto group, and lactonisation of ring D; the latter is the first report of this reaction by a Rhizopus species. Fusarium lini promoted 1-dehydrogenation of the steroid, which has been rarely observed in fungi cultures. The other routes of biotransformations included oxidation of the 17beta-hydroxyl group and the hydroxylation at 11alpha position. These reactions are not common for Fusarium species.