Cells of the marine fish pathogen Vibrio anguillarum 775 harboring a plasmid associated with virulence can grow unaffected in the presence of iron-binding compounds such as transferrin. In contrast, the growth of isogenic plasmidless derivatives is inhibited by the presence of iron chelators. Radioactive from (55Fe3+) uptake experiments indicate that this plasmid-linked ability of V. anguillarum cells to grow under conditions of iron limitation is indeed due to a more rapid and efficient iron uptake mediated by the virulence plasmid. In addition, V. anguillarum cells growing under iron limitation show at least two novel outer membrane proteins. One of them, a 86,000-dalton protein we called OM2, is inducible only in those cells in which the virulence plasmid is present.