When cyanobacterium cells are grown under extremely low CO(2) concentration, the number of carboxysomes, structures containing ribulose-bisphosphate carboxylase (Rubisco; EC 22.214.171.124), is known to increase. This suggests that Rubisco helps to regulate photosynthesis in cyanobacteria. However, no studies have been done on the changes of Rubisco content and activity in response to the extracellular CO(2) concentration, and no information is available on its effect on photosynthesis. To elucidate the relationship between the expression responses of Rubisco and extracellular CO(2), wild-type cells (Synechococcus PCC7942) and carboxysome-lacking cells were grown under various CO(2) concentrations, and Rubisco activity was determined. In both strains, Rubisco activity increased when the cells were grown under a CO(2) concentration around, or less than, K (1/2)(CO(2)) of photosynthesis. In carboxysome-lacking cells, Rubisco activity increased five to six times at most, and a simultaneous increase in the rate of photosynthesis was observed. These results suggest that stimulation of expression of Rubisco occurs to compensate for the decrease in the rate of photosynthesis under CO(2)-limited conditions.