The par locus from the plasmid pSC101 and the parB (hok/sok) locus from the R1 plasmid have been used to enhance plasmid stability in Escherichia coli. To compare the effects of the par and the parB loci on plasmid stability and host cell growth, four plasmids (pPT, pPTP, pPTB, and pPTBP) which are different only in terms of the presence or absence of par, parB, or par/parB were constructed. While the plasmid pPT without any plasmid stabilization locus was very unstable under a condition in which trp alpha protein was not produced, the plasmids pPTP, pPTB, and pPTBP carrying par, parB, and both par and parB, respectively, were shown to be stably maintained over 100 generations under the same condition. Under the repressed condition, the four recombinant strains harboring each plasmid did not show distinct differences in growth rates. However, under a trp alpha protein overproduction condition, the pPTB-bearing strain showed poor growth compared to the other recombinant strains. This suggests that, when the intrinsic plasmid loss rate is high, the parB locus alone may not be as useful as the par locus for maintaining unstable plasmids and increasing overall productivity of a recombinant protein.