The industrial production of the antibiotic Thylosin based on the cultivation of filamentous Streptomyces fradiae, is considered as an example of an analysis of possible oxygen deficiency caused by a growth-induced rise of apparent viscosity. The time-course of broth theology and its power law pseudoplastic behaviour are illustrated. The stirred vessel hydrodynamics is represented by mass balances derived on the basis of the ’network-of-zones’ concept developed previously. The axisymmetrical stirred bed is divided into backmixed cells interrelated by liquid fluxes carrying a content of gas. Bubble size, liquid circulation I-ate, bubble rise velocity and gas flow rate are the model parameters. The effects of theology are introduced in the model by relationships known to describe bubble motion due to buoyancy and due to liquid circulation, both depending on apparent viscosity. By this means, the local gas hold-up distribution patterns are obtained and related to theology numerically. Zones of non-uniform gas distribution are identified and related to viscosity variation. Referring to the sample fermentation, the results are intended to form the basis of a reasonable process policy.