The reduction of ethyl 4-chloro-3-oxobutanoate (COBE) to ethyl (R)-4-chloro-3-hydroxybutanoate (R-CHBE) biocatalysed by the aldehyde reductase expressed from E.coli M15 (pQE30-alr) in combination with regeneration of NADPH by the glucose dehydrogenase expressed from E.coli M15 (pQE30-gdh) is reported.The reaction in buffer solution was optimized with regard to high productivity (up to 36.8 g/(L d)) and enantioselectivity (>99.9%).