The ftsZ gene of Escherichia coli, which lies in a cluster of cell division genes at 2 min on the genetic map, codes for a protein which is thought to play a key role in triggering cell division. Using an ftsZ::lacZ operon fusion, we have studied the transcription of the ftsZ gene under conditions in which cell division was either inhibited or synchronized in the bacterial population. In ftsZ, ftsA, ftsQ, and ftsI (or pbpB) mutants, there was no change in the differential rate of expression of the ftsZ gene in nonpermissive conditions, when cell division was completely blocked. Although the FtsZ protein is thought to be limiting for cell division, in synchronized cultures the ftsZ gene was expressed not only at the moment of septation initiation but throughout the cell cycle. Its expression, however, was not exponential but linear, with a rapid doubling in rate at a specific cell age; this age, about 20 min after division in a 60-min cycle, was different from the age at which the ftsZ::lacZ operon was duplicated. However, it was close to the age at which replication initiated and at which the rate of phospholipid synthesis doubled. During the transient division inhibition after a nutritional shift-up, ftsZ transcription again became linear, with two doublings in rate at intervals equal to the mass doubling time in the rich medium; it adopted the exponential rate typical of rich medium about 60 min after the shift-up, just before the bacterial population resumed cell division. The doubling in the rate of ftsZ transcription once per cycle in synchronized cultures and once per mass doubling time during the transition period after a nutritional shift-up reflects a new cell cycle event.