An experimental study of rheological behavior of three morphological types of fermentation broths in relation to process conditions was carried out in industrial and pilot-plant bioreactors. Three industrially manufactured antibiotic broths were studied : Cephalosporin C (fungal), tylosin (actinomycetic), and apramycin (proactinomycetic), in which the rheology may cause real problems.A viscometer with coaxial cylinders and six-blade impeller was used for rheological measurements. The shear rates range in which both the instruments give similar results was determined.During the experiments rheological and morphological behavior, pH, activity, biomass concentration, fats, glucose, and nitrogen concentrations were studied.All the broths studied exhibited nonNewtonian behavior, which could be described with Ostvald-de-Waele power-law model.It was found that the rheological behavior of the broths during the process is closely related to biomass concentration and morphological changes of mycelia, so that viscosity could be used as a parameter for process monitoring and regulation. A set of experiments on oxygen-transfer improvement in the case of the most viscous tylosin broths by water addition was also done.