Continuous flow bioreactors with recombinant bacteria sometimes exhibit spontaneous sustained oscillations. The possibility and the conditions for such oscillations in large reactors are not known precisely. In this study are investigated these conditions with a model of incomplete mixing in large bioreactors. The dilution rate for oscillations is a nonlinear function of the mixing characteristics. Such a dependence limits the translation of inferences from oscillations in small well-mixed vessels to larger reactors, but also enables suitable control of the feed rate to large bioreactors either to regulate or to suppress oscillations.