Scale-up of bialaphos production from a 3 l jar fermentor to a 300 kl fermentor was investigated. When the scale-up was based on power consumption (for agitation) per unit volume of the reactor, bialaphos production in the 300 kl fermentor was remarkably lower than that obtained with the 3 l jar fermentor. The reason for the unsuccessful scale-up was investigated by considering the major differences between the 3 l jar and the 300 kl fermentor. It was found that variation in the shear stress had no significant effect on bialaphos production. A special pressure-proof fermentor with pressure control system was used to study the effect of liquid pressure gradient in the 300 kl fermentor. When the cells in the special fermentor were subjected to cyclic variation in pressure (between 1,013 and 1,994 HPa) as though they are circulated between the top and the bottom of a 10 m liquid level in the 300 kl fermentor, bialaphos production was as low as that obtained in the 300 kl fermentor. The optimum DO concentration for bialaphos production in the 3 l fermentor was found to be 0.5 ppm and above this concentration, an inverse relationship between the DO concentration and bialaphos production was observed. The inhibition of bialaphos production at high pressure was therefore attributed to the increase in the DO concentration at high liquid pressures. When the scale-up was based on the DO concentration and the DO at the middle of the 300 kl fermentor was controlled at 0.5 ppm, bialaphos production increased to 85 % of that obtained with the 3 l jar fermentor. By taking the DO concentration gradient in the 300 kl fermentor into consideration, the DO at the bottom of the fermentor was maintained at 0.5 ppm. This resulted in a successful scale-up wherein bialaphos production increased to 96% of the maximum production in the 3 l jar fermentor.