A filtration rig equipped with a tubular alumina membrane was used to study the performance of crossflow microfiltration of Lactobacillus helveticus. Experiments were performed at constant permeation flux. High cell concentrations and fast transient conditions to the stationary J adversely affected permeability. Membrane fouling was due to a fast irreversible layer formation and to a reversible cell cake. This microbial deposit characteristics were dependent on the ratio permeation flux/wall shear stress, J/tau(w). Fouling was faster and more severe when J/tau(w) was greater than a critical value of 1.15 L(-1) . h(-1) . m(-2) . Pa-1. The disordered structure of this cell cake seemed to lead to a macromolecule deposit between the cells which adversely affected the membrane permeability.