The reduction of ethyl 3-oxobutanoate (EOB) to ethyl (S)-3-hydroxybutanoate ((S)-EHB) by cell suspensions of baker's yeast, (Saccharomyces cerevisiae) in a continuously operated reactor with cell retention has been investigated. The continuous reductions were technically feasible and reproducible. High degrees of conversion (91-98%) and high extents of reduction (88-92%) were obtained. High product concentrations were not achievable due to impractical long residence times needed. In a continuously operated stirred-tank reactor containing 26-31 g dw l(-1) of baker's yeast and achieving a product concentration of similar to20 mM, the overall reduction rate (0.14-0.17 mmol product g dw(-1) h(-1)) remained constant in time indicating the robustness of baker's yeast as biocatalyst at the process conditions applied. The volumetric productivity, which was not optimized, was 0.64 g l(-1) h(-1). Due to the maintained low EOB concentration, the enantiomeric excess of (S)-EHB remained high (>98%) during the first 24 h. However, in spite of the low residual EOB level, the enantiomeric excess value decreased slowly to 80-92% in a few days. This is probably due to an increasing level of intrinsic activity in the yeast towards the undesired (R)-enantiomer. Long-term production of (S)-EHB in a continuous process, therefore, seems to be hindered by this change in enantioselectivity of the biocatalyst.