The aim of this experimental work was to study a cell-recycle membrane fermentor for the production of lactic acid. Initially, the fermentation and the ultrafiltration processes were studied separately; afterward, they were combined to obtain a hybrid system. Batch fermentation lasted for about 25 h, giving a yield of about 62% and a productivity of about 0.43 g/(L h). The fermented broth coming from these experiments was used to characterize the polyamide (PA) and polysulfone (PS) membranes with different nominal molecular-weight cutoffs used for the filtration step. The behavior of the permeate flux as a function of transmembrane pressure was investigated. The hydraulic resistance of membrane and fouling components was evaluated. The highest contribution to the reduction of flux was due to reversible fouling. This is because the experiments were carried out at a low axial velocity (in the range between 0.35 and 0.650 m/s) so that the cells would not be damaged and aeration would not occur (as this is an anaerobic fermentation). Although the PA membranes were fouled by adsorption, the relative flux reduction was lower for this type of membrane (0.90) than for PS 100 kDa (0.93) and PS 0.1 mum (0.97). Furthermore, it was found that the cells in the exponential growth phase had higher fouling effects compared to those in the dead phase. The efficiency of the hybrid system as a whole was appraised in terms of bacterial growth, lactic acid concentration, productivity, production yield, and flux decline with time.