Abstract: In this study we have examined ptxR expression in the P. aeruginosa strain (PA103ΔXR) which displays a unique phenotype. The strain was derived from PA103-2R which contains two copies of ptxR and was constructed by the integration of a ptxR plasmid in the PA103 chromosome. PA103ΔXR was isolated by continuously subculturing PA103-2R in antibiotic free media and screening for carbencillin sensitive colonies. Under iron deficient medium, toxA transcription is enhanced significantly. Two peaks of toxA transcription were detected at 4 and 16 h post inoculation. In contrast, toxA transcription in PA103ΔXR was significantly reduced. In iron-sufficient medium toxA transcription was repressed in both PA103 and PA103ΔXR. ptxR regulates toxA expression through the regAB locus. Under iron deficient conditions regA transcription was enhanced after 8 h of growth and reached a peak at 16 h of growth. This was followed by a sharp reduction in regA transcription during 20 and 24 h of growth In contrast, regA transcription in PA103ΔXR was significally reduced at 12 and 16 h of growth. Under high iron conditions, the patterns of regA transcription in PA103 and PA103ΔXR appears to be similar In both strains, it is significantly reduced. The unique phenotype of PA103ΔXR is not due to mutations in either toxA, regAB and ptxR. A plasmid carrying intact toxA, regAB and ptxR failed to complete the defect of PA103ΔXR in exotoxin A synthesis.