The transition metal manganese is considered to be a minor micronutrient in both pro- and eukaryotes, usually being required from the environment at subnanomolar levels. Until recently, Mn was only known to function in cells as a cofactor for a few enzymatic reactions. A notable exception has been reported in many lactic acid bacterial species which require micromolar medium Mn levels for growth and contain up to 35 mM Mn. These high Mn concentrations are accompanied by the near or complete absence of intracellular iron and superoxide dismutase (SOD). Lacking hemes, Lactobacillus plantarum and related species contain a unique Mn-cofactored catalase as well as millimolar Mn(II) in a nonenzymic complex performing the function of the micromolar superoxide dismutase found in most other aerotolerant cells. The high Mn(II) levels are accumulated via an efficient active transport system and are stored intracellularly in a high molecular weight complex. Study of Lactobacillus plantarum has provided an interesting example of the substitution of Mn for Fe in several of the biological roles of Fe, an alternative mechanism of aerotolerance, and a better understanding of the unique biochemistry of the lactic acid bacteria.