Continuous-flow column reactors were used to study the dynamics of plasmid exchange in a structured, thermodynamically open system containing either Enterobacter cloacae or Pseudomonas cepacia, both carrying the transmissible recombinant plasmid R388::Tn1721. Plasmid transfer rates were higher in vermiculite and sterile soil columns supplied with nutrient solution than those in sterile and non-sterile soil columns without input of nutrient solution. For both species, donor and recipient strains took about 5 days to reach their maximum densities in effluents from the columns supplied with nutrient solution. After about 8 day s the donor and transconjugant populations of P. cepacia in the effluent solution decreased exponentially, whereas E. cloacae donor, recipient and transconjugant strains maintained steady-state concentrations. The difference between plasmid stability in the two species may have significant consequences in terms of releasing plasmid-bearing genetically modified microorganisms into the natural environment. The plasmid is persistent in E. cloacae in non-sterile soil even though its transfer to the marked recipient in non-sterile soil was minimal.