AbstractNatural transformation is the mechanism of procaryotic gene transfer that involves the uptake and expression of genetic information encoded in extracellular DNA. This process has been regarded as a mechanism to transfer genes (primarily chromosomal markers) between closely related strains or species. Here we demonstrate the cell–contact–dependent transfer of a non–conjugative plasmid from a laboratoryE. colistrain to a marineVibriospecies, the first report of intergeneric natural plasmid transformation involving a marine bacterium. The nucleic acid synthesis inhibitors nalidixic acid and rifampicin inhibited the ability of theE. colito function as a donor. However, dead cells also served as efficient donors. There was an obligate requirement for cell contact. No transfer occurred in the presence of DNase I, when donors and recipients were separated by a 0.2–μm filter, or when spent medium alone was used as a source of transforming DNA. These results indicate that contact–mediated intergeneric plasmid exchange can occur in the absence of detectable viable donor cells and that small non–conjugative plasmids can be spread through heterogeneous microbial communities by a process previously not recognized, natural plasmid transformation. These findings are important in the assessment of genetic risk to the environment, particularly from wastewater treatment systems and the use of genetically engineered organisms in the