We describe here the development of a new approach to the analysis of Escherichia coli replication control. Cells were grown at low growth rates, in which case the bacterial cell cycle approximates that of eukaryotic cells with Gl, S, and G2 phases: cell division is followed sequentially by a gap period without DNA replication, replication of the single chromosome, another gap period, and finally the next cell division. Flow cytometry of such slowly growing cells reveals the timing of replication initiation as a function of cell mass. The data show that initiation is normally coupled to cell physiology extremely tightly: the distribution of individual cell masses at the time of initiation in wild-type cells is very narrow, with a coefficient of variation of less than 9%o. Furthermore, a comparison between wild-type and seqA mutant cells shows that initiation occurs at a 10-20% lower mass in the seqA mutant, providing direct evidence that SeqA is a bona fide negative regulator of replication initiation. In dnaA(Ts) mutants the opposite is found: the mass at initiation is dramatically increased and the variability in cell mass at initiation is much higher than that forwild-type cells. In contrast to wild-type and dnaA(Ts) cells, seqA mutant cells frequently go through two initiation events per cell division cycle, and all the origins present in each cell are not initiated in synchrony. The implications for the complex interplay amongst growth, cell division, and DNA replication are discussed.Normally, cells growing under steady-state conditions duplicate their chromosomal complement once and only once between each cell division. In any given steady-state culture of Escherichia coli cells, initiation of DNA replication at the chromosomal origin, oriC, occurs at a specific time in the cell cycle and at a specific cell mass. Two questions regarding the relationship between cell growth and initiation are of particular importance.First, how is the timing of initiation coupled to cell growth? An early hypothesis suggested that the initiation process responds to cell mass per se (1). According to this model, initiation always occurs at a fixed ratio of mass to origins-i.e., the initiation mass is a constant which is independent of the growth conditions. More recent evidence suggests, however, that the situation is more complicated, since the experimentally determined initiation mass varies significantly with growth rate (2, 3).Second, how variable is the mass at the time of initiation, when the cells grow under steady-state conditions? The extent of variation indicates the tightness of coupling between initiation and cell physiology, of which cell mass is an indicator: the less the variation, the tighter the coupling.Appropriately controlled replication initiation requires the assembly of the replication machinery at oriC. A central component of this assembly is the initiator protein DnaA.