Edwardsiella ictaluri, the etiological agent of enteric septicemia of channel catfish (ESC) is the leading cause of bacterial disease in commercially raised channel catfish Ictalurus punctatus. The only drug approved by U.S. Food and Drug Administration for use against ESC is Romet-30. Recently, several isolates were obtained that had a naturally occurring resistance to Romet-30. On further characterization these isolates were shown to possess a 55-kilobase (kb) plasmid that encodes resistance to the drug. We compared Romet-30-resistant E. ictaluri and a Tribrissen-resistant strain of Escherichia coli (strain 1898) isolated from equine cystitis. Antimicrobial profiles, plasmid screening, restriction digest, and Southern blot analysis indicated that the two plasmids are very similar. The resistance afforded to the E. coli was encoded by a 55-kb plasmid. Each of the R plasmids conferred resistance to Romet-30, tetracycline, and Terramycin (oxytetracycline). The R plasmid from E. coli strain 1898 was transferred to Romet-30-sensitive isolates of E. ictaluri by single-step conjugation, rendering the transconjugates resistant to Romet30, tetracycline, and Terramycin. Each plasmid was cleaved into fragments by restriction enzymes, then electrophoresed in an agarose gel and transferred to nitrocellulose. A labeled probe prepared from the R plasmid of E. ictaluri isolate 5-90-156, one of the original Romet-30-resistant E. ictaluri isolates, was added. Restriction enzyme digestion resulted in an equal number of fragments of equal mobility for the two plasmids, and the probe hybridized with the same fragments of each plasmid. These results indicate that the R plasmids of E. ictaluri and E. coli are similar, if not identical.