To clarify the physiological function of pseudovitamin B(12) in cyanobacteria, we determined pseudovitamin B(12) contents and cobalamin-dependent methionine synthase activity in Synechocystis sp. PCC6803 grown under CoSO(4)-sufficient and -limited conditions. Pseudovitamin B(12) and cobalamin-dependent methionine synthase activity (0.8 nmol/min/mg protein) were found in a homogenate of the Synechocystis cells grown for 10 d in the CoSO(4)-sufficient medium. The cellular pseudovitamin B(12 )contents increased significantly at the early logarithmic growth phase and thereafter decreased rapidly at the stationary phase; a similar fluctuation pattern was shown in the cobalamin-dependent methionine synthase activity. Although the CoSO(4)-limited conditions did not reduce the cell growth, pseudovitamin B(12) contents and methionine synthase activity decreased significantly in the limited cells relative to the sufficient cells. These results indicate that the cyanobacterium Synechocystis sp. PCC6803 can synthesize pseudovitamin B(12) de novo and utilize it as the cofactor for cobalamin-dependent methionine synthase.