The optimum dilution rate for 2,3-butanediol (BDL) production by Paenibacillus polymyxa was 0.2 h(-1) and the optical purity of BDL remained above 98 % at all dilution rates. With decreasing culture pH, ethanol and BDL production increased, whereas the optical purity of BDL decreased to 94 % at pH 5.7. In the chemostat culture at pH 6.3 and a 0.1 h(-1) dilution rate, the optimum air supply for BDL production was 200 ml min(-1) in which the O-2 uptake rate was 6.7 mmol l(-1) h(-1). Under this condition, the optical purity of BDL decreased to 93 %.