While the volumetric productivity of lactic acid increased in continuous culture of Lactobacillus casei with cell recycle, enhanced formation of by-products such as acetate, formate, ethanol, and D-lactate was observed in the cell-recycled fermenter compared with a simple chemostat at a similar range of dilution rate. The increased formation of by-product which was significantly dependent on substrate limitation resulted from a lower dilution rate rather than a high cell concentration in the cell-recycled fermenter.