Mercury resistance plasmids were exogenously isolated, i.e., recovered after transfer to a model recipient bacterium, from marine air-water interface, bulk water, and biofilm communities during incubation in artificial seawater without added nutrients. Ninety-five plasmids from different environments were classified by restriction endonuclease digestion, and 12 different structural plasmid groups were revealed. The plasmid types isolated from different habitats and from different sampling occasions showed little similarity to each other based on their restriction endonuclease patterns, indicating high variation and possibly a low transfer between microhabitats and/or a different composition of the microbial communities at different sites and times. With another approach in which probes derived from one of the isolated plasmids and a mercury resistance (mer) probe from Tn501 were used, similarities between plasmids from several different groups were found. The plasmids were further tested for their incompatibility by use of the collection of inc/rep probes (B/

This review summarises the literature on bacterial gene transfer in marine ecosystems. Relevant experiments carried out in model systems are also included. Prerequisites for the main gene transfer mechanisms, transformation, transduction and conjugation are discussed, such as concentrations of extracellular DNA in marine waters, numbers of bacteriophages in sea water and frequency of plasmids in marine bacteria. Transfer of chromosomal genes as well as plasmids are considered. We also discuss the possibility that gene transfer is more frequent in surface‐associated bacterial communities. Examples of relevant studies using various solid surfaces and from the air‐water interface are summarized. We suggest that there is a higher ‘flow‐rate’ of genetic information through surface‐associated communities compared to bulk water communities.