The effects of phosphate supply and aeration on cell growth and PHB accumulation were investigated in Azotobacter chroococcum 23 with the aim of increasing PHB production. Phosphate limitation favoured PHB formation in Azotobacter chroococcum 23, but inhibited growth. Azotobacter chroococcum 23 cells demonstrated intensive uptake of orthophosphate during exponential growth. At the highest phosphate concentration (1.5 g/litre) and low aeration the amount of intracellular orthophosphate/g residual biomass was highest. Under conditions of fed-batch fermentation the possibility of controlling the PHB production process by the phosphate level in the cultivation medium was demonstrated. A 36 h fed-batch fermentation resulted in a biomass yield of 110 g/litre with a PHB cellular concentration of 75% dry weight, PHB content 82.5 g/litre, PHB yield Y-P/S = 0.24 g/g and process productivity 2.29 g/litre.h.