Escherichia coli produces two distinct species of catalase, hydroperoxidases I and II, which differ in kinetic properties and regulation. To further examine catalase regulation, a lacZ fusion was placed into one of the genes that is involved in catalase synthesis. Transductional mapping revealed the fusion to be either allelic with or very close to katE, a locus which together with katF controls the synthesis of the aerobically inducible hydroperoxidase (hydroperoxidase II). katE was expressed under anaerobic conditions at levels that were approximately one-fourth of those found in aerobically grown cells and was found to be induced to higher levels in early-stationary-phase cells relative to levels of exponentially growing cells under both anaerobic and aerobic conditions. katE was fully expressed in air and was not further induced when the growth medium was sparged with 100% oxygen. Expression of katE was unaffected by the addition of hydrogen peroxide or by the presence of additional lesions in oxyR or sodA, indicating that it is not part of the oxyR regulon. When katF::Tn10 was introduced into a katE::lacZ strain, beta-galactosidase synthesis was largely eliminated and was no longer inducible, suggesting that katF is a positive regulator of katE expression.