<P><B>ABSTRACT</B><P> NAD-dependent l - and d -lactate dehydrogenases coexist in Lactobacillus genomes and may convert pyruvic acid into l -lactic acid and d -lactic acid, respectively. Our findings suggest that the relative catalytic efficiencies of <I>ldhL</I> - and <I>ldhD</I> -encoded products are crucial for the optical purity of lactic acid produced by Lactobacillus strains. </P></P>