Chlorella pyrenoidosa Chick catalyzed the reduction of methyl 3-oxopentanoate (Me 3-oxoPen) to the corresponding (S)-(+)-3-hydroxy ester in 60% enantiomeric excess (ee). An improvement of the ee of (S)-(+)-3-hydroxy ester was achieved by increasing substrate concentration and by the addition of 1% metal salts, namely, NaCl and KCl (75-78% ee). No shift of ee toward the (R)-(-)3-hydroxy ester was observed following this addition. The addition of allyl alcohol brought about a decrease in both the chemical yields and the ee of (S)-(+)3-hydroxy ester. Differences in the ability to reduce Me 3-oxoPen were observed among three strains of Chlorella, of which C. pyrenoidosa Chick yields the (S)-(+)-3-hydroxy ester but C. regularis and C. vulgaris Beijerinck yield (R)-(-)-enantiomer. In particular, C. regularis transformed Me 3-oxoPen to (R)-(-)-3-hydroxy ester in 79% ee, with a 10% yield.