Cultures of Rhizopus arrhizus in complex or defined media in shake-flasks were found to form pelleted growth morphologies. When polymeric substances which increased medium viscosity were added, cultures grew in a more dispersed form. The most effective polymers were those which could bind divalent cations from solutions. These polymers also reversed the severe pelleting of cultures observed when magnesium ions were added. It was considered that this property explained, at least in part, how the polymers affected culture morphology. Viscosity was not the only factor affecting dispersal of R. arrhizus cultures. Dispersed cultures produced higher biomass levels generally. This may have been due to the more efficient mass-transfer of essential nutrients to the biomass.