One of the major concerns in the utilization of plasmid-containing organisms in fermentation processes is the stability of the plasmid. In general, it is observed that two parameters, the probability of plasmid loss due to faulty segregation and the difference in the specific growth rate between the recombinant and reverted cells, govern the dynamics of the reactor. These parameters are affected by a number of factors: the genetic make-up of the host cells and the plasmid and the reactor operating parameters such as temperature, pH, dilution rate, and growth medium. It is speculated that by manipulating these factors it may be possible to enhance the stability of the plasmid-containing population. In this work the effects of well-defined perturbations in reactor conditions on plasmid maintenance and expression of a population of Escherichia coli K12 strain containing the plasmid pBR322 grown in a chemostat with a non-selective medium were studied. In a set of experiments the cultures were exposed to square-wave oscillations in the dilution rate. Under these conditions the cultures were capable of maintaining a mixed population of plasmid-containing and plasmid-free cells for a longer period of time than a culture grown under a constant dilution rate. The effect of the perturbations on plasmid copy number and expression were also studied. The data was dependent on the amplitude and frequency of the oscillations. In addition, the experiments indicate that adaptation of the culture to the transient conditions will reduce or totally eliminate any advantage created by the perturbations. An unstructured model, evaluating the probability of plasmid loss and the differences in the specific growth rate between the recombinant and reverted cells grown under dilution rate oscillations, was developed to describe the experimental data. The results indicate that the transient conditions provide a favorable environment for the plasmid-containing population. It was further observed, through the use of minimum inhibitory concentration experiments, that the plasmid-containing cells existed as a heterogeneous population with respect to the plasmid copy number. This population distribution could play an important role in plasmid stability and expression.