The effect of aeration during cell growth on the subsequent reduction of 2-hexanone and 2-octanone by yeast cells entrapped in calcium alginate beads was studied. The reactions were conducted using a-propanol as a sacrificial substrate to regenerate the cofactor NAD(H), and a mixture of (S)- and (R)-alcohols was produced. The use of strictly aerobic conditions when growing the cells resulted in the highest initial reaction rates, as well as the production of only a single product (i.e., the enantiomeric excess of the (S)-alcohols was 1.0) However, initial reaction rates decreased proportionally with fermentation time regardless of whether the yeast were grown aerobically or under both aerobic and anaerobic conditions. The data also suggest that it is the aerobic (or anaerobic) condition, rather than the cell growth phase, which is responsible for the results seen.