The effect of aeration rate on pullulan production and the theological properties of the fermentation broth in an airlift reactor was investigated. An airlift fermenter was shown to be an appropriate fermentation system for the production of pullulan. A maximum pullulan concentration (30kgm(-3)), biomass concentration (6.0kgm(-3)), pullulan yield (60%, w/w) and sugar utilization (100%, w/w) was obtained at an aeration rate of 2vvm. The mycelium and the yeast-like cells were the morphological forms responsible for pullulan production. The highest polysaccharide concentration was obtained when the mycelial forms and the yeast-like cells were 60% and 40% (w/w) of the total biomass, respectively. The apparent viscosity of the broth was increased with the increase of the aeration rate from 1 to 2vvm and then decreased at higher vvms. On the other hand, the dissolved oxygen concentration and the volumetric mass transfer coefficient continually increased with the increase of the aeration rate. The mycelial forms and the production of extracellular polysaccharide were responsible for the non-Newtonian flow behaviour of the fermentation broth. The theological behaviour can be characterized by a power law type of equation. The relationship between shear ratel shear stress and shear rate/apparent viscosity showed a non-Newtonian behaviour of the fermentation broth.