S. ABBAD‐ANDALOUSSI, E. GUEDON, E. SPIESSER AND H. PETITDEMANGE. 1996. Glycerol catabolism by Clostridium butyricum DSM 5431 into acetate, butyrate and 1,3‐propanediol (1,3‐PD) was studied in chemostat culture. The fact that the intracellular concentrations of NADH (18–22 μUmol g‐1dry cell mass) were extremely high suggested that the dehydratase activity was the rate limiting step in 1,3‐PD formation. This limitation was proved by the addition of propionaldehyde, another substrate of propanediol dehydrogenase, into the culture medium. This resulted in an increase in (i) glycerol utilization, (ii) biomass formation and (iii) product biosynthesis.