Rheological properties of fermentation broth during polysaccharide biosyntesis by Aureobasidium pullulans in bioreactor were examined. The fermentation process in synthetic broth containing saccharose was carried out by 96 hours. Agitator speeds from 200 to 1000 rpm were applied to obtain various cultivation conditions in the bioreactor. Rheological characteristics of the broth during fermentation process and under various process conditions were analyzed. The Newtonian behaviour of the broth was noted only during the first day after inoculation. Then the broth attained the non-Newtonian pseudoplastic fluid characteristics. The Ostwald-de-Weale equation was used to describe rheological properties of the cultivation medium. The constant presenting the non-Newtonian behaviour of the fluid, the consistency and apparent viscosity of the fermentation broth were calculated. The highest apparent viscosity increase was observed during the second day of fermentation process. Increased agitation caused increase of the apparent viscosity. Rheological properties of the medium were compared with those of water solutions of pure pullulan at concentrations from 1 to 5%. The solutions were Newtonian and the viscosity was several times lower then the consistency of the broth. It was found that the structure formed by biopolymers and other metabolites such as melanins determine the apparent viscosity of cultivation medium.