The problem of designing a saturated Output-Feedback (OF) controller for a continuous bioreactor with Haldane kinetics is addressed. The reactor must be operated at maximum biomass production rate by manipulating the feed rate on the basis of a (biomass or substrate) measurement. The consideration of the problem as an interlaced observer-control design leads to a saturated PI controller that recovers (up to observer convergence) the behavior of a detailed model-based robust nonlinear state-feedback (SF) globally stabilizing saturated controller. The saturated PI control scheme has: (i) closed-loop stability conditions in terms of control gains and limits, and (ii) a simple construction-tuning procedure. The proposed approach is illustrated and tested with a representative case example through numerical simulations. (C) 2011 Elsevier Ltd. All rights reserved.