Batch fermentations of Rhizobium trifolii 0403 were performed with dissolved oxygen (DO) control to determine the effect of DO on growth rate, and the production of exopolysaccharide and biomass. When DO was maintained between 4 and 20% of saturation, growth was exponential. The apparent specific growth rate, biomass and exopolysaccharide production did not vary significantly over this DO range. At oxygen transfer rates less than the minimum required for exponential growth, DO fell to below 1%. Growth under these conditions was linear and in direct relation to the oxygen transfer rate. Exopolymer production increased and biomass production decreased under oxygen-limited conditions.