To test the hypotheses that eubacterial genomes leave evolutionarily stable structures and that the variety of genome size is brought about through genome doubling during evolution, the genome structures of Haemophilus influenzae, Mycoplasma genitalium, Escherichia coli, and Bacillus subtilis were compared using the DNA sequences of the entire genome or substantial portions of genome. In these comparisons, the locations of orthologous genes were examined among different genomes. Using orthologous genes for the comparisons guaranteed that differences revealed in physical location would reflect changes in genome structure after speciation. We found that dynamic rearrangements have so frequently occurred in eubacterial genomes as to break operon structures during evolution, even after the relatively recent divergence between E. coli and H. influenzae. Interestingly, in such eubacterial genomes of high plasticity, we could find several highly conservative regions with the longest conserved region comprising the S10, spc, and alpha operons. This suggests that such exceptional conservative regions have undergone strong structural constraints during evolution.