Raw glycerol, the main by-product of the bio-diesel production process, was converted to 1,3-propanediol by Clostridium butyricum F2b. In batch cultures, 47.1 g dm(-3) of 1,3-propanediol were produced. Continuous cultures were conducted at a constant dilution rate (=0.04h(-1)) and various inlet glycerol concentrations with 1,3-propanediol produced at levels up to 44.0 g dm(-3). At increasing glycerol concentrations in the inlet medium, biomass yield decreased. This decrease was attributed to the microbial metabolism being directed towards the biosynthesis of organic acids (and hence carbon losses as CO2) instead of biochemical anabolic reactions. An autonomous analytical model was developed, and quantified the effect of inlet glycerol concentration on the production of biomass and metabolites. Indeed, high inlet substrate concentrations positively affected the biosynthesis, principally of butyric acid and to a lesser extent that of acetic acid. In contrast, at increased glycerol concentrations, the relative increase of 1,3-propanediol production per unit of substrate consumed was lower as compared with that of acetic and, mainly, butyric acid. This could be explained by the fact that the butyric acid pathway represents an alternative and competitive one to that of 1,3-propanediol for re-generation of NADH(2) equivalents in the microbial cell. (C) 2004 Society of Chemical Industry.