A reversed two-environment model has been used to study micromixing in continuous fermentation for growth and product formation by recombinant bacteria. As an example, an Escherichia coli M72 strain harbouring the plasmid pPLc23trpA1 and producing tryptophan synthetase is considered. With excess substrate, 10% plasmid-free bacteria in the initial broth do not affect the steady-state concentrations of plasmid-containing bacteria but their mass fractions decrease significantly. With a smaller substrate concentration, the mass fractions decrease sharply as the dilution rate increases if micromixing is good; but concentrations still remain high. There is also a clear demarcation between regions of good and poor micromixing. These results are explained and an optimal combination of micromixing and dilution rate is suggested to maximise productivity.