The continuous fermentation of 1,3-propanediol from glycerol by Clostridium butyricum was subjected to cell recycling by filtration using hollow-fibre modules made from polysulphone. The performance of the culture system was checked at a retention ratio (dilution rate/bleed rate) of 5, dilution rates between 0.2 h(-1) and 1.0 h(-1) and glycerol input concentrations of 32 g l(-1) and 56 g l(-1). The near-to-optimum propanediol concentration of 26.5 g l(-1) (for 56 g l(-1) glycerol) was maintained up to a dilution rate of 0.5 h(-1) and then decreased while the propanediol productivity was highest at 0.7 h(-1). The productivity could be increased by a factor of four in comparison to the continuous culture without cell recycling. By application of the model of Zeng and Deckwer [(1995) Biotechnol Prog 11 : 71-79] for cultures under substrate excess, it was shown that the limitations resulted exclusively from product inhibition and detrimental influences from the cell recycling system, such as shear stress, were not involved.