Pyoverdine is the main siderophore secreted by fluorescent pseudomonads to scavenge iron in the extracellular environment. Iron uptake, however, needs to be tightly regulated, because free iron stimulates the formation of highly toxic oxygen derivatives. In the opportunistic pathogen Pseudomonas aeruginosa, the transcriptional regulator OxyR plays a key role in the upregulation of defense mechanisms against oxidative stress as it stimulates the expression of the antioxidant genes katB, ahpB and ahpCF after contact with oxidative stress-generating agents. Inactivation of the oxyR gene in Pseudomonas fluorescens ATCC 17400 and in P. aeruginosa PAO1 impairs pyoverdine-mediated iron uptake. The pyoverdine utilization defect can be restored by complementation with the oxyR gene of P. aeruginosa, as well as by adding catalase. Growth of the oxyR mutant in low- or high-iron media is also impaired at a low, but not at a high inoculum density. Uptake of radioactive (59)Fe pyoverdine is, however, not affected by the oxyR mutation, nor is the transcription of the fpvA gene encoding the ferripyoverdine receptor, suggesting that the defect lies in the inability to remove iron from the ferrisiderophore.