1,3-Propanediol inhibition during glycerol fermentation to 1,3-propanediol by Clostridium butyricum CNCM 1211 has been studied. The initial concentration of the 1,3-propanediol affected the growth of the bacterium more than the glycerol fermentation. mu(max) was inversely proportional to the initial concentration of 1,3-propanediol (0-65 g l(-1)). For glycerol at 20 g l(-1) the growth and fermentation were completely stopped at an initial 1,3-propanediol concentration of 65 g. However, for an initial 1,3-propanediol concentration of 50 g l(-1) and glycerol at 70 g l(-1), the final concentration (initial and produced) of 1,3-propanediol reached 83.7 g l(-1) (1.1 M), with complete consumption of the glycerol. Therefore, during the fermentation, the strain tolerated a 1,3-propanediol concentration higher than the initial inhibitory concentration (65 g l(-1)). The addition of 1,2-propanediol or 2,3-butanediol (50 g l(-1)) in the presence of glycerol (50-100 g l(-1)), showed that 2-diols reduced the mu(max) in a similar way to 1,3-propanediol. The measurement of the osmotic pressure of glycerol solutions, diols and diol/glycerol mixtures did not indicate any differences between these compounds. The hypothesis of diol inhibition was discussed. Taking into account the strain tolerance of highly concentrated 1,3-propanediol during fermentation, the fermentation processes for optimising production were considered.