The first example of immobilized baker's yeast in alginate fibers with double-layer mediated reduction of ketones is described. The asymmetric reduction of ethyl benzoylformate (EBF) to ethyl (R)-mandelate ((R)-EM) in a continuous process was carried out in a packed bed reactor with Saccharomyces cerevisiae immobilized in calcium alginate fibers with double gel layers. The reaction was performed under aerobic conditions and glucose feeding as electron donor. The immobilization procedure prevents cell leakage from alginate fibers into the medium, in a device in which the alginate fibers cells were restricted to the inner layer, whereas the outer layer helped to prevent leakage and contact with potential inhibitors. High chemical yield (82%) and extents of reduction (92-97%) were obtained in operating conditions that we optimized by changing the concentration of EBF and feed flow, obtaining volumetric productivity of 0.12 mmol /(L h) and enantiomeric excess (ee) of (R)-EM, which remained high (92%) during eight days of continuous operation. (c) 2005 Elsevier Inc. All rights reserved.