Abstract: The ability of Nostoc ellipsosporum PTCC 1659, a cyanobacterium strain, for biotransformation of androst-4-en-3,17-dione (AD) was studied. Fermentation was performed in BG-11 medium supplemented with 0.05% AD at 25°C for seven days incubation. The single metabolite obtained was purified using chromatographically methods and characterized as testosterone on the basis of its spectroscopic features. Bioreaction characteristic observed was 17-carbonyl reduction. Time course study showed the accumulation of the product from the third day of the fermentation and reached to the maximum in the seventh day. Production of testosterone was not affected by aeration. Continuously illumination or 16 h light/8 h dark has no effect on the transformation yield. Optimum concentration of the substrate, which gave maximum bioconversion efficiency, was 0.5 mg mL-1 in one batch. Biotransformation was completely inhibited in a concentration above 2.0 mg mL-1.