The growth characteristics of Synechococcus PCC7942 for carbon dioxide (CO2) fixation was studied. When the culture was illuminated at 8 klx and sparged with 5% CO2 (volume/volume), high CO2 uptake rate (CUR) of this strain appeared. In the batch reactor under above condition, the highest values for both the specific growth rate and final cell mass concentraion were achieved by the pH controled cultivation at pH 6.8. This condition also led to the highest CUR on the second day of the cultivation. Therefore, based on these observations, the additional cultivation of Synechococcus PCC7942 for keeping the highest CUR value was performed in chemostat (1.1 dm(3)). This experiment achieved the result that the cell suspension of Synechococcus PCC7942 per 1 dm(3) of culture in chemostat took in 3.6 g of CO2 for six days.