Dynamic responses during start-up performance and following perturbations in single process parameters (dilution rate and feed sugar concentration) were studied in a continuous stirred tank bioreactor employing S. cerevisiae for the bioconversion of sugars to ethanol. The attainment of the steady state at the startup of the bioreactor was found to be a function of feed sugar concentration and dilution rate. A third-degree polynomial fit resulted in the theoretical computation of the number of reactor volumes necessary to attain a steady state. Step changes in the feed sugar concentration or the dilution rate showed overshoots and undershoots in the specific growth and ethanol formation rates. The system performance was dependent on the input noise and the operating conditions. Increased productivity could be obtained during the transition phase. The dynamic behaviour was simulated by making a non-steady-state mass balance. The simultaneous differential equations were solved by the fourth-order Runge-Kutta method. Theoretical analysis adequately described the dynamic response.