Citrobacter freundii DSM 30040 immobilized on modified polyurethane carrier particles PUR 90/16 was used for continuous glycerol fermentation in an anaerobic fixed bed reactor with effluent recycle and pH control (fixed bed loop reactor). The fermenter was run with buffered mineral medium under growth conditions resulting in the permanent renewal of active biomass. The effects of glycerol concentration in the feed, dilution rate (D), pH and temperature (T) were investigated to optimize the process. With 400 mM glycerol in the feed, pH 6.9, T= 36 degrees C and D = 0.5 h(-1) the maximum productivity could be determined as 8.2 g/l per hour of 1,3-propanediol.