Several environmental factors affecting the succinic acid production by Anaerobiospirillum succiniciproducens were studied. External CO2 supply had a negative effect on cell growth at both pH 6.2 and pH 6.5, while it had a somewhat positive effect on succinic acid production. The effect of CO2 on succinic acid yield was higher at pH 6.5 than pH 62. External supply of H-2 enhanced the cell growth rate and succinic acid yield. The optimum ratio of H-2/CO2 was 5:95% (v/v) when 20 g l(-1) glucose was used. Under this condition, succinic acid yield and productivity were 91% and 1.8 g l(-1) h(-1) , respectively The formation of the byproduct was not much influenced by the external supply of H-2. The distribution of the fermentation products was not affected by varying the initial glucose concentration in a reasonable range, but the succinic acid yield and productivity were higher at 40 g l(-1) glucose. It was concluded that the external supply of CO2 and H-2 at the optimum ratio makes it possible to produce succinic acid with a high yield and productivity.