ABSTRACT A yellow-pigmented marine bacterium, designated strain SD-21, was isolated from surface sediments of San Diego Bay, San Diego, Calif., based on its ability to oxidize soluble Mn(II) to insoluble Mn(III, IV) oxides. 16S rRNA analysis revealed that this organism was most closely related to members of the genus Erythrobacter , aerobic anoxygenic phototrophic bacteria within the α-4 subgroup of the Proteobacteria (α-4 Proteobacteria ). SD-21, however, has a number of distinguishing phenotypic features relative to Erythrobacter species, including the ability to oxidize Mn(II). During the logarithmic phase of growth, this organism produces Mn(II)-oxidizing factors of ≈250 and 150 kDa that are heat labile and inhibited by both azide and o -phenanthroline, suggesting the involvement of a metalloenzyme. Although the expression of the Mn(II) oxidase was not dependent on the presence of Mn(II), higher overall growth yields were reached in cultures incubated with Mn(II) in the culture medium. In addition, the rate of Mn(II) oxidation appeared to be slower in cultures grown in the light. This is the first report of Mn(II) oxidation within the α-4 Proteobacteria as well as the first Mn(II)-oxidizing proteins identified in a marine gram-negative bacterium.