The performance of continuous fermentation for an induced recombinant protein depends on the concentration of the inducer and on the inflow stream. The latter is characterized by the substrate concentration and the dilution rate, of which the latter is preferred as the manipulated variable for control. This work studies the effects of inducer (isopropyl-beta -D-thiogalactopyranoside, IPTG) concentration and dilution rate on the steady state performance of a continuous fermentation for P-galactosidase production by Escherichia coli containing the plasmid pUR2921. A low concentration of IPTG favours the growth of both viable (recombinant) cells and nonviable cells. To promote preferential growth of the former, their mass fraction should be maximized, and this requires a larger, optimal IPTG concentration. The expressed concentration of beta -galactosidase passes through maxima with respect to both IPTG concentration and the dilution rate, whereas the viable cell concentration attains a peak only with respect to the dilution rate. While the optimal IPTG concentrations for mass fraction and for beta -galactosidase are widely different, the two dilution rates are not far apart. This suggests two-stage fermentation at an intermediate dilution rate and different concentrations of IPTG.