Using the heat pulse technique, the local mean ow liquid velocity and the mixing conditions for twophase¯ow in the riser of an airlift bioreactor have been measured and analysed. Xanthan-gum solutions were used as the physical model to some ®lamentous broths reported in the literature. A two-fold decrease of liquid velocity and diffusional mixing regime are predicted for the course of a fermentation process proceeding in a non-Newtonian biomass growth circulation system.