The mycobacterial IdeR protein is a homologue of the diphtheria-toxin repressor DtxR. We have previously demonstrated that Mycobacterium tuberculosis IdeR, like DtxR, represses transcription of Corynebacterium diphtheriae iron-regulated promoters in vivo and binds to C. diphtheriae operators in a metal-dependent manner in vitro. We show here that ideR mutants of M. smegmatis, constructed by allelic replacement, were defective in their ability to repress siderophore biosynthesis in the presence of iron. They were also more sensitive to hydrogen peroxide and had decreased levels of catalase/peroxidase (KatG) and manganese superoxide dismutase (Mn-SOD). This indicates that IdeR is a negative regulator of siderophore production and is required for the response to superoxide- and hydrogen peroxide stress. We propose that IdeR is the mycobacterial counterpart of the Escherichia coli Fur protein, i.e. it is a pleiotropic regulator that couples iron metabolism to the oxidative-stress response.