We showed that two very different manganese transporters occur in various important genera of marine bacteria. The ABC transporter encoded by sitABCD of the model Roseobacter-clade bacterium Ruegeria pomeroyi DSS-3 is required for Mn(2+) import and was repressed by the Mur (Manganese uptake regulator) transcriptional regulator in Mn-replete media. Most genome-sequenced Roseobacter strains contain SitABCD, which are in at least two sub-groups, judged by their amino-acid sequences. However, a few Roseobacters, for example, Roseovarius nubinhibens, lack sitABCD, but these contain another gene, mntX, which encodes a predicted inner membrane polypeptide and is preceded by cis-acting Mur-responsive MRS sequences. It was confirmed directly that mntX of Roseovarius nubinhibens encodes a manganese transporter that was required for growth in Mn-depleted media and that its expression was repressed by Mur in Mn-replete conditions. MntX homologues occur in the deduced proteomes of several bacterial species. Strikingly, all of these live in marine habitats, but are in distantly related taxonomic groups, in the γ- and α-proteobacteria. Notably, MntX was prevalent in nearly all strains of Vibrionales, including the important pathogen, Vibrio cholerae. It also occurs in a strain of the hugely abundant Candidatus Pelagibacter (SAR11), and in another populous marine bacterium, Candidatus Puniceispirillum marinum (SAR116). Consistent with this, MntX was abundant in marine bacterial metagenomes, with one sub-type occurring in an as-yet unknown bacterial clade.