A series of high-copy-number Escherichia coli expression vectors equipped with an oxygen-sensitive promoter Pvgb of Vitreoscilla hemoglobin ( encoded by the vgb gene) were constructed and characterized. Plasmid pKVp containing P-vgb was inducible by low oxygen tension, while plasmid pKVp containing a partition ( par) region from plasmid pSC101 ligated to P-vgb provided inheritable stability for the vectors in the absence of ampicillin. Plasmid pKVpV had the Vitreoscilla hemoglobin operon vgb ligated to P-vgb, while a construct containing P-vgb, the vgb operon and a par region constituted plasmid pKVpPV. Shake- flask studies demonstrated that plasmids pKVpV and pKVpPV expressed higher levels of Vitreoscilla hemoglobin under low aeration condition ( 5% air saturation in water) compared with the levels observed under strong aeration ( 20% air saturation in water). Introduction of either the enhanced green fluorescent protein ( eGFP) gene egfp or the toluene dioxygenase ( TDO) gene tod into either pKVpV ( P-vgb, vgb operon) or pKVpPV ( Pvgb, vgb operon, par) slightly attenuated (similar to 30%) the strong expression of VHb under low aeration. However, all displayed approximately a three- fold increase versus that observed for strong aeration. Recombinant E. coli harboring either pKVp- E ( P-vgb, egfp) or pKVpP- E ( P-vgb, par, egfp) displayed at least a two- fold increase in eGFP expression under conditions of low aeration and absence of antibiotic, compared with that under strong aeration after 24 h of cultivation. Strong expression of TDO was also observed using low aeration in recombinant E. coli harboring pKVpPV- T ( P-vgb, vgb operon, par, tod) or pKVpP-T ( Pvgb, par, tod). Plasmids containing the par region were stable over 100 generations. These results indicate that the novel expression system combining plasmid stability over the cell growth phase and a promoter inducible by low oxygen tension will be very useful for high- density production of foreign proteins.