The metabolism of C. butyricum was manipulated, at neutral pH and in carbon limited chemostat cultures by changing the overall degree of reduction of the substrate, using mixtures of glucose and glycerol. Cultures grown on glucose alone produced only acids (acetate, butyrate and lactate). When the glycerol (in C moles)/glucose+glycerol (in C moles) ratio was progressively changed from 0 to 1 a corresponding increase of 1,3-propanediol production occured and an immediate and drastic decrease of the specific rate of acetate production was observed while the specific rate of butyrate production only decreased slightly. For glycerol (in C moles)/glucose+glycerol (in C moles) ratios higher than 0.5, the q(NAD(P)H from Fd) and the CO2/H-2 molar ratio increased sharply, the first becoming positive and the second higher than 1. This indicates a complete reversion of the electron flow : part of reduced ferredoxin produced by the phosphoroclastic cleavage of pyruvate to acetyl-CoA was diverted from H-2 formation toward NAD(P) reduction by the ferredoxin-NAD(P) reductase(s) in order to produce NAD(P)H. This change in the electron flow was associated to an increase in the specific rate and the yield of 1,3-propanediol production related to glycerol.