The virulence of the fish pathogen Vibrio anguillarum 775 is mediated by the pJM1 plasmid-specified iron uptake system which is expressed under conditions of iron limitation. Other V. anguillarum strains isolated from various geographical locations harbor plasmids that are highly related to pJM1 and that are also associated with the high-virulence phenotype of these strains. In this work, we found that a pJM1-like plasmid, pJHC1, from one of these virulent strains encoded an iron uptake system that resulted in an increased level of production of the siderophore anguibactin. The gene(s) responsible for increased anguibactin production was included within the iron uptake region of plasmid pJHC1. The cloned iron uptake regions of pJHC1 and pJM1 possessed identical restriction endonuclease maps, suggesting that the DNA region encoding those genes in pJHC1 may have diverged subtly from that in pJM1. Analysis of the iron uptake system from other V. anguillarum strains carrying pJM1-like plasmids demonstrated that strains originating from diseased fish from the Atlantic coast carry plasmids encoding an increased-siderophore-production phenotype, while strains isolated from Pacific Ocean locations behaved as the 775 strain.