Actinomycetes, especially members of the genus Streptomyces, are responsible for producing the majority of known antibiotics. The production of antibiotics by filamentous organisms is often dependent on the morphology and size distribution of the pellet population within the culture. Particle interaction and subsequent pellet formation are primarily dependent on the rate of collision of particles in culture, which is in turn, a function of fluid turbulence. The microbial polysaccharide xanthan gum was used to artificially regulate the apparent viscosity (mu(a)) of S. hygroscopicus fermentation broths with the aim of controlling particle interaction, aggregation and hence pellet formation. An increase in both pellet count and biomass concentration from approximately 2,000 to 8,000 pellets ml(-1) and 0.9 - 2.1 gl(-1) dry weight of biomass, as well a decrease in the mean pellet volume from 0.014 to 0.004 mm3 was observed in cultures supplemented with 3 gl(-1) xanthan gum. The addition of xanthan gum significantly alters fluid rheology by increasing the mu(a). Counter-intuitively, an increase in the mu a within the experimental range examined resulted in an increase in the rate of gas - liquid mass transfer. This was attributed to the predominantly diffusive nature of oxygen transfer in shake flask cultures.