AbstractContinuous‐flow column reactors were used to study the dynamics of plasmid exchange in a structured, thermodynamically open system containing eitherEnterobacter cloacaeorPseudomonas 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.cepaciain the effluent solution decreased exponentially, whereasE. cloacaedonor, 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.cloacaein non‐sterile soil even though its transfer to the marked recipient in non‐sterile soil