Plasmid pJM1 from an invasive strain of Vibrio anguillarum mediates an iron-sequestering system that is associated with the ability of this bacterium to cause septicemia in marine fishes. This plasmid-mediated iron uptake system was analyzed by using mutations caused by transposon Tnl. Restriction endonuclease analysis of iron uptake-deficient and -proficient derivatives generated by insertion of Tnl and molecular cloning experiments permitted us to localize the plasmid regions involved in the process of iron sequestration to a stretch of about 20 kilobase pairs. In addition, the existence of two plasmid-mediated components involved in the process of iron uptake in V. anguillarum was defined: a diffusible substance which functions as a siderophore and a nondiffusible receptor for complexes of iron-siderophore, which we have tentatively identified as the pJM1 plasmid-mediated outer membrane protein OM2 of V. anguillarum.