A series of thirty marine and estuarine bacterial isolates was examined for the ability to naturally transform with plasmid DNA. One isolate from Tampa Bay, Florida, identified as Vibrio parahaemolyticus, successfully incorporated and maintained the broad host range plasmid pKT230 in both filter transformation assays and sterile sediment microcosms, at frequencies ranging from 0.3 to 3.1 x 10 to the minus eighth power transformants per recipient. Transformation occurred without deletions and transformation of multimeric forms of the plasmid. Results suggest that natural transformation may be one mechanism by which estuarine bacteria acquire antibiotic resistance plasmids.