Pseudomonas aeruginosa exotoxin A (ETA) production depends on the virulence-factor regulator Vfr. Recent evidence indicates that the P. aeruginosa iron-starvation sigma factor PvdS also enhances ETA production through the ETA-regulatory gene regA. Mutants defective in vfr, regA and pvdS, plasmids that overexpress these genes individually and lacZ transcriptional/translational fusion plasmids were utilized to examine the relationship between vfr, regA and pvdS in regulating P. aeruginosa ETA production. ETA concentration and regA expression were reduced significantly in PAODeltavfr, but pvdS expression was not affected. Overexpression of Vfr produced a limited increase in ETA production in PAODeltapvdS, but not PAODeltaregA. Additionally, overexpression of either RegA or PvdS did not enhance ETA production in PAODeltavfr. RT-PCR analysis showed that iron did not affect the accumulation of vfr mRNA in PAO1. These results suggest that: (i) Vfr enhances toxA expression in PAO1 both directly and indirectly through regA, but not through pvdS; (ii) vfr expression is not regulated by iron; and (iii) both Vfr and PvdS cooperate in the presence of RegA to achieve a maximum level of toxA expression.