Two hundred and sixty strains of microorganisms, (60 strains of yeasts, 60 strains of bacteria and 140 strains of fungi), were evaluated for their ability to stereoselectively bioreduce a beta-ketoester {[2S-(2 alpha,4 beta)]-1-[(1,1-dimethylethoxy)carbonyl]-4-hydroxy-beta-oxo-2-pyrrolidinepropanoic acid 1,1-dimethylethyl ester} to the corresponding (R)-beta-hydroxyester {[2S-(2a(S*),4 beta)]-B,4-dihydroxy-1-[(1,1-dimethylethoxy)carbonyl]-2-pyrrolidinepropanoic acid 1,1-dimethylethyl ester}, a precursor to the beta-methyl carbapenem antibiotic BO 2727. Among all the microbes evaluated, only one fungal strain, Mortierella alpina MF 5534 (ATCC 8979) was found to catalyze the desired reaction. The scaled-up bioconversion process in laboratory bioreactor (23-l scale) supported (R)-beta-hydroxyester titers of 550 mg/l during a 250-h cultivation cycle and allowed the timely production of gram quantities of diastereomerically pure materials (diastereomeric excess>98%).