A method of determining control coefficients and pseudo-first-order kinetic parameters is presented for cell growth, substrate consumption, and product formation in a chemostat bioreactor. From the equations that relate the control coefficients to process variables, such as feed concentrations of the limiting substrate and the product of interest, a perturbation method is developed to determine control coefficients from steady-state measurements. This method combined with the transient response analysis provides a practical way for determining control coefficients and estimating kinetic properties ill chemostat bioreactor systems. To determine the kinetic parameters, one measures cell mass, concentrations of the limiting substrate and the product of intel est following a step change in the feed concentration until the system reaches a new steady state. The rime courses of these variables are processed to obtain control coefficients, which yield the pseudo-first-order kinetic constants for cell growth, substrate consumption, and product formation. Only steady-state responses are needed in calculation, ifa step change in cell concentration in tile feed stream cart also be introduced without significantly perturbing cell physiology. This method is useful in characterizing the kinetics of whole cell bioreactions : results from chemostat experiments call be used to design operating strategies for batch or fed-batch bioreactions. It is generally applicable to continuous-stir-tank reactors with interacting parallel reactions.