Salmonella typhimurium excreted the rhamnose fermentation product 1,2-propanediol not only under anaerobic conditions, but also under aerobic conditions. The absence of an aldehyde dehydrogenase enzymatic activity that oxidizes to lactate the lactaldehyde formed in the dissimilation of rhamnose raised the intracellular concentration of the aldehyde which was alternatively reduced to the excretable 1,2-propanediol by a residual propanediol oxidoreductase activity.