The time of replication of a bacterial plasmid, Flac, during the division cycle of Escherichia coli has been estimated in exponentially growing cultures and at various times after a shift from minimal medium to a richer medium (a shift-up). There is a variation in the cell age at which the capacity to synthesize beta-galactosidase (beta-D-galactoside galactohydrolase, EC 3.2.1.23) doubles (assumed to be a measure of the time at which the Flac plasmid replicates) when this capacity is measured at various times during the shift-up, and with increasing steady-state exponential growth rate. Cells growing at slow and moderate growth rates exhibit Flac replication in the middle of the division cycle. With increasing time after a shift-up or with increasing growth rate the plasmid replicates at earlier times, eventually at cell division, and finally in the older cells. This variation in the cell age at which the plasmid replicates is similar to the variation in cell age at which chromosome initiation occurs during a shift-up, although plasmid replication occurs slightly before initiation of chromosome replication.