This paper proposes a robust nonlinear observer for bioreactors combining the classical asymptotic observer and a nonlinear Luenberger-like observer. The resulting hybrid observer considers a new definition of the hybridization parameter which reflects the kinetic model confidence. The nonlinear observer is tuned on the basis of robust H-infinity approach and the differential-algebraic representation (DAR) of nonlinear systems. A simulated case study concerning fed-batch animal cell cultures is presented to demonstrate the potentials and advantages of the proposed approach for state estimation of bioreactors. (C) 2013 Elsevier Ltd. All rights reserved.