In the continuous membrane fermentation of Lactobacillus rhamnosus at biomass concentrations above 100 g/L the viscosity of the system increased sharply resulting in decreased permeate flux. At cell densities higher than 108 g/L a slight deviation from Newtonian behaviour was observed. Steady state was achieved and main kinetic characteristics remained constant for around 100 hours when the biomass concentration was maintained at a constant level by continuous bleeding.