The design of control laws for bioprocesses is generally based on simplified single-species models. Biodiversity is nonetheless inherent in any bioreactor where contamination leads to a mixture of different species or strains. This paper proposes to define and study the robustness to biodiversity of bioprocess control laws: given a control law designed for one species, what happens when additional species are present? is the approach is illustrated with a well-used control law which regulates substrate concentration using measurement of growth activity. Depending on the properties of the additional species, the control law can lead to the required objective, but also to an undesired monospecies equilibrium point, coexistence, or even a failure point. Finally, for this case, the robustness can be improved by a saturation of the control. Robustness to biodiversity is a difficult issue which should be better understood and accounted for in the control design. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 2742-2750, 2017