AbstractThree feedback strategies for the on‐line control of cell densities in a mixed‐culture system have been examined. A competitive mixed‐culture system ofCandida utilisandCorynebacterium glutamicumgrown on glucose as the limiting carbon source was modeled using Monod growth kinetics. First‐order time constants were added to simulate transient growth effects. Multivariable feedback control of cell densities by manipulation of substrate feed and dilution rate was investigated. Feedback strategies directed to minimizing control interactions were found to be superior to classical feedback. Transients in the growth‐rate response produced oscillations in cell density and required retuning of control constants. The relative time constants of the two species were important, with the largest oscillations resulting when the faster growing organism had the faster time