In this paper, we propose to regulate the output of an auto-catalytic bioprocess (a biological process associated with a growth of a micro-organism) by means of a recirculation loop and by-pass. We give conditions on the volume of the reactor and the kinetic function for which it is possible to stabilize the output concentration under a constant or disturbed unmeasured input flow. Furthermore, we obtain convergence of the output variable in finite time with a Lipschitz continuous feedback. Key words: Continuous bioreactor, recirculation loop, output regulation, nonlinear control design.