A putative operon encoding an uncharacterized ferrous iron transport (FtrABCD) system was previously identified in cDNA microarray studies. In growth studies using buffered medium at pH values ranging from pH6.0 to 7.6, Bordetella pertussis and Bordetella bronchiseptica FtrABCD system mutants showed dramatic reductions in growth yields under iron-restricted conditions at pH6.0, but had no growth defects at pH7.6. Supplementation of culture medium with 2mM ascorbate reductant was inhibitory to alcaligin siderophore-dependent growth at pH7.6, but had a neglible effect on FtrABCD system-dependent iron assimilation at pH6.0 consistent with its predicted specificity for ferrous iron. Unlike Bordetella siderophore-dependent and haem iron transport systems, and in agreement with its hypothesized role in transport of inorganic iron from periplasm to cytoplasm, FtrABCD system function did not require the TonB energy transduction complex. Gene fusion analysis revealed that ftrABCD promoter activity was maximal under iron-restricted growth conditions at acidic pH. The pH of human airway surface fluids ranges from pH5.5 to 7.9, and the FtrABCD system may supply ferrous iron necessary for Bordetella growth in acidic host microenvironments in which siderophores are ineffective for iron retrieval.