ABSTRACT In Lactococcus lactis , the interactions between oxidative defense, metal metabolism, and respiratory metabolism are not fully understood. To provide an insight into these processes, we isolated and characterized mutants of L. lactis resistant to the oxidizing agent tellurite (TeO ₃ ²⁻ ), which generates superoxide radicals intracellularly. A collection of tellurite-resistant mutants was obtained using random transposon mutagenesis of L. lactis . These contained insertions in genes encoding a proton-coupled Mn ²⁺ /Fe ²⁺ transport homolog ( mntH ), the high-affinity phosphate transport system ( pstABCDEF ), a putative osmoprotectant uptake system ( choQ ), and a homolog of the oxidative defense regulator spx ( trmA ). The tellurite-resistant mutants all had better survival than the wild type following aerated growth. The mntH mutant was found to be impaired in Fe ²⁺ uptake, suggesting that MntH is a Fe ²⁺ transporter in L. lactis . This mutant is capable of carrying out respiration but does not generate as high a final pH and does not exhibit the long lag phase in the presence of hemin and oxygen that is characteristic of wild-type L. lactis . This study suggests that tellurite-resistant mutants also have increased resistance to oxidative stress and that intracellular Fe ²⁺ can heighten tellurite and oxygen toxicity.