The use of a new pulsing device, Elastic Membrane Pulsator (EMP), 1 coupled to a packed-bed bioreactor has proved useful for increasing the efficiency of several lab-scale fermentation processes. 2,3 With the aim of studying the applicability of this system at a larger scale, a semipilot pulsing packed-bed bioreactor (2.1 l, HM D = 2.7 ) equipped with several sensors (pH, T, gas and liquid flow meters) was employed to carry out the ethanolic fermentation of glucose using Saccharomyces cerevisiae immobilized in Ca 2+-alginate. This fermentation process was selected as a model. Glucose concentrations of 100 and 200 g l −1 and dilution rates between 0.34 and 1.45 h −1 were chosen. The effect of pulsation frequencies between 0.024 and 0.008 Hz is analyzed and compared with the results from the system without pulsation. The results show that the performance of the system can be improved by up to 18% when pulsations at suitable frequencies are applied. However, when pulsation is produced asynchronously with gas production, an important backmixing was observed especially in the upper zone of the fermentor. This implies a loss of efficiency; therefore, the selection of the proper pulsation characteristics appears to be a key factor in the stability and performance of this system. The bioreactor performance has been modeled considering the apparent kinetics, external mass transfer, and hydraulics (flow model) in order to better understand the various observed behavior.