Succinic acid was produced by continuous fermentation of Actinobacillus succinogenes sp. 130Z in an external membrane cell recycle reactor to improve viable cell concentration and productivity. Using this system, cell concentration increased to 16.4 g/l at the dilution rate 0.2 h(-1), up to 3 times higher than that of batch culture, and the volumetric productivity of succinic acid increased up to 6.63 g/l/h at the dilution rate 0.5 h(-1), 5 times higher than that of batch fermentation. However, in the continuous culture using a high dilution rate, operational problems including severe membrane fouling and contamination by lactic acid producer were observed. Another succinic acid producer, Mannheimia succiniciproducens MBEL55E, was also utilized in this system, and the cell concentration and productivity of succinic acid at the dilution rate of 0.3 h(-1) were found to be above 3 and 2.3 times higher, respectively, compared with those obtained at the dilution rate of 0.1 h(-1). These observations give a deep insight into the process design for a continuous succinic acid production by microorganisms.