Mass production of Poly(beta-hydroxybutyrate) (PHB) from Metylobacterium organophilum under potassium-limited condition was carried out using a microcomputer-aided automatic fed-batch culture system. The concentration of methanol was kept within the range of 2 - 3 g/l which did not show any inhibitory effect on cell growth. The PHB accumulation was stimulated when potassium concentration in the culture broth fell below 25 mg/l. After 70 hours of cultivation the concentrations of cell mass and PHB were obtained to be 250 g/l and 130 g/l, respectively, which corresponded to a volumetric productivity of 1.8 - 2.0 g-PHB/1-hr. PHB contents ranged from 52% to 56% of dry cell weight with a yield factor (Y-P/S) Of 0.19 g-PHB/g-methanol.