In rapidly growing bacteria, replication cycles overlap and the cells contain replisomes assembled at different generations. We have compared the organization of replication forks in cultures with different cell cycles, strains and labeling techniques. The extent of fork-colocalization was found to be dependent on the replication pattern. In slowly growing cells, sister forks were found to colocalize for a fraction of the replication time, whereas cells growing with higher overlapping of replication cycles showed an extensive grouping of replication forks. These results indicate that clustering of replisomes correlates with the number of rounds of replication harbored by each chromosome. The dynamic organization of replisomes reported here, may allow ending the controversy among alternative models. Cell division, DNA-to-cell mass ratio and the integrity of the replication factories were compromised in a ribonucleotide reductase mutant. These results reinforce the idea that regulatory networks connect the synthesis of DNA precursors with the cell cycle. (C) 2008 Elsevier Ltd. All rights reserved.