An actinomycete (MA 6474, ATCC 53828) isolated from a soil sample (Mutare, Zimbabwe) was found to biotransform the sodium salt of Simvastatin (MK-733) to 6-alpha-hydroxymethyl MK-733, 6-beta-hydroxymethyl MK-733, and 6-ring-hydroxy MK-733. The bioconversion efficiency to the desired compound, 6-alpha-hydroxymethyl MK-733, was enhanced by optimizing the physico-chemical parameters of the process. In shake flask cultures, addition of magnesium (0.125 mg/l Mg SO4.7H2O) to the medium resulted in a five-fold increase in the rate of bioconversion to the alpha diastereomer. The ratio of bioconversion products (6-alpha-hydroxymethyl,6-beta-hydroxymethyl, and 6-ring-hydroxy MK-733) was regulated by pH. Process improvements and scale up in 23-1 fermentors, which consisted of a controlled addition of substrate (MK-733), resulted in a 2-fold increase in alpha diastereomer production (42 vs. 79 U/ml) and a 23-fold rate increase in the formation of alpha-diastereomer. A high diastereomeric ratio (alpha: beta = 9:1) facilitated downstream processing.