A gene highly homologous to the Escherichia coli dnaA gene, essential for initiation of chromosomal replication, is conserved in evolutionarily diverged bacteria, Bacillus subtilis, Pseudomonas putida and Micrococcus luteus. In addition, a unique repeating sequence, DnaA-box, present in E. coli oriC and recognized by DnaA protein, is clustered in noncoding regions flanking these putative dnaA genes. These results suggests that interaction between DnaA protein and DnaA- box is key step in the initiation of bacterial chromosomal replication in common. Some preliminary results concerning evolution of regulation of chromosomal replication are also discussed.