Rheological properties and oxygen mass transfer coefficient (k(L) a) were investigated in a stirred reactor (10 dm(3)) in the course of fermentations producing microbial polysaccharides-pullulan and xanthan. The fermentation broths behaved as pseudoplastic non-Newtonian fluids in both cases. Studies on the relationship between fluid theological properties and k, a were also carried out. The oxygen mass transfer coefficient decreases during the fermentation and exponential equations have been obtained to describe the relationship between the oxygen mass transfer coefficient, the agitation speed and the apparent viscosity of the broths. Furthermore, comparison of results between pullulan and xanthan fermentations was investigated. For the xanthan fermentation process, mixing and mass transfer in the reactor were more difficult than those for the pullulan fermentation.