Abstract A high-cell-concentration culture of Propionibacterium acidi-propionici in a fermenter with recycling through a tangential membrane filtration unit is described. Circulation rate and fermentation start-up procedures are discussed in order to optimize the system performance and the operation at high dilution rates. Maximum cell concentrations up to 140 g dry weight/l were obtained, with a propionic acid productivity of 10.3 g/l hr and a product yield of 0.56 g propionic acid/g glucose. Rheological studies showed that at cell mass concentrations above 90-100 g/1 the fermentation broth viscosity increased rapidly and changed its behaviour from Newtonian to pseudoplastic. The use of an adequate cell bleed rate allowed the cell concentration to be maintained at a desirable value and consequently to stabilize the reactor operation.