In this paper I shall confine myself to only one aspect of chromosome replication in bacteria: its control and co-ordination with growth and cell division. The nature of the problem to be considered is made clear by two features of chromosome replication in
Escherichia coli
, First, under conditions of rapid growth, involving generation times of up to about one hour,
DNA
synthesis is essentially continuous; there is no detectable resting period corresponding to the
G
period typically found in higher organisms. Secondly, in glucose minimal media, as the data of Cairns (1963) and others have shown, a single replication point, or growth point, traverses the length of the chromosome during each cycle of replication. It follows that, although the rate of replication in
E. coli
might be determined by the supply of
DNA
precursors, the maintenance of the proper
sequence
of events cannot be controlled in this way since in a system in which
DNA
synthesis is continuous these precursors must be present at all times. Under the conditions mentioned above in
E. coli
, for example, the cell must have some means of ensuring that a new cycle of replication is not initiated until the previous one is complete. Consequently the important point of control of replication in bacteria must be over the initiation of replication rather than replication itself.