Fermentation and microfiltration characteristics of D-lactic acid producing bacterium were evaluated in order to provide fundamental understanding of the optimization of fermentation process.Production of lactic acid by Lactobacillus delbrueckii was influenced by pH, and efficient production was observed at pH 5-7.In microfiltration, filtration behaviors of L.delbrueckii were markedly influenced by pH of culture broth.According to the measurement of the zeta potential of L.delbrueckii, it was clarified that electrostatic interaction between cells led to aggregation of cells at lower pH.These results indicate that the determination of culture condition based upon both characteristics of fermentation and filtration of bacteria is extremely important for the optimization of fermentation process.