The efficiency of an ion exchange system coupled to a bioreactor to extract on-line inhibitory organic acids produced was evaluated. Batch fermentations without dialysis and fed-batch experiments with and without dialysis of Propionibacferiumfueudenreichii were conducted. It was possible to keep the propionic acid concentration in the reactor at a low level to avoid complete growth inhibition. Improvements in biomass and propionic acid productivities were achieved when the dialysis system was used. The performance of the dialysis system was improved when a new pH control strategy evaluated in this study was used.