The kinetics of cell division have been studied in a strain of Escherichia coli which has an amber mutation in the ftsA gene and which also carries a temperature sensitive amber suppressor. This strain is therefore temperature sensitive for the synthesis of the ftsA protein. Cells of this strain were able to divide only if the synthesis of this protein took place during a specific part of the cell cycle. This was a short period (roughly 10 min in duration) immediately before the normal time of cell division.