The rheological properties of the fermentation broth during pullulan production from synthetic medium byAureobasidium pullulansP56 in a stirred tank fermentor were investigated. The mycelial forms and the production of extracellular polysaccharide were responsible for the non‐newtonian flow behaviour of the fermentation broth. The rheological behaviour can be characterized by a power law type of equation. The relationship shear rate/shear stress and shear rate/apparent viscosity showed a non‐newtonian behaviour of the fermentation broth. The power law index was found to be a sensitive factor for the indication of the pseudoplastic behaviour of the broth.