In a model system composed of a synthetic lake water, plasmid R68.45 was shown to transfer between P. aeruginosa strains at frequencies between 10 to the minus third power and 10 to the minus fifth power. Transconjugants were detected about 100 times more frequently at the neuston than in the bulk water, which reflected a greater enrichment of the donor at this site. None of the aquatic isolates were able to act as recipients of plasmid R68.45 in the model system with PAO4032 as donor. The results suggest that the spread of R68.45 or similar plasmids into this particular aquatic population through lateral transfer under nutrient deprived conditions would be a rare event. This does not, however, exclude the possibility that less copiotrophic aquatic forms may be more effective recipients under these conditions or that once established, R68.45, and other similar plasmids might proliferate through the neuston.