Cells of Synechococcus sp., a rod-shaped, unicellular cyanobacterium (blue-green alga) can be readily snychronized by depriving the cells of carbon dioxide and light for a 12 h period. On resumption of growth, a portion of the population undergoes two sharply synchronized divisions. Ribonucleotide reductase activity was found to be maximal during the time of DNA synthesis in these cells. The peak of reductase activity could be abolished by adding inhibitors such a chloramphenicol to the culture, suggesting that the enzyme is induced at the gene level in the cyanobacteria. Additional properties of ribonucleotide reductase were investigated in Synechococcus cells made permeable by treatment with ether. Cytidine triphosphate reduction is absolutely dependent on adenosylcobalamin (coenzyme B₁₂) and is subject to allosteric stimulation by deoxyadenosine triphosphate.