Production of L-ornithine under dual limitation of arginine and phosphate was investigated in a chemostat culture of an arginine auxotroph of Brevibacterium ketoglutamicum. Under an arginine-limited condition, culture instability, including the formation of revertant cells, could be a serious problem due to the inherent instability of the auxotrophic mutant strain. The difference in the growth rates between the auxotrophs and revertants was considered to be the main cause of the culture instability, and dual Limitation with arginine and phosphate was found most appropriate to overcome this problem. When the ratio of the phosphate to arginine concentration in the feed medium was varied between 0.55 and 5.22 at a fixed dilution rate of 0.1 h(-1), three distinct growth phases were recognized : phosphate-limited (0.55 to 2.01), dual arginine- and phosphate-limited (2.01 to 2.16) and arginine-limited (above 2.16). In the dual arginine- and phosphate- limitation phase, the culture stability was markedly improved while the yield of L-ornithine was maintained at a high level. When the dilution rate was varied from 0.04 h(-1) to 0.17 h(-1) under the dual-limitation condition, maximum L-ornithine productivity was obtained at a value of 0.27 g/l/h at D=0.1 h(-1).