The effect of dilution rate on the instability kinetics of Escherichis coli RV308(pHSG415) during glucose-limited continuous culture is examined. Two nonlinear models are fitted to the data, both of which characterize the plasmid-host system in terms of the rate parameters R (for the plasmid segregation rate) and dp (for the specific growth rate difference between plasmid-free and plasmid-bearing single cells). In the first model, both R and dp have constant values with respect to time. In the second, either R or dp is represented as a time-dependent function. Although both models fit the data equally well, it is demonstrated that the constant rate parameter model gives results which appear to be misleading. A comparison is also made among some of the many plasmid instability models (both mass-balance and segregated) which have appeared in the literature. It is found that all of these give identical trajectories and differ only in the definitions of the rate parameters used.