BACKGROUND: Polyhydroxybutyrate (PHB) is a biopolymer with thermo-mechanical properties like those of plastics produced by the petrochemical industry. These properties are improved in PHBs of high or ultra-high weight average molecular weight. Azotobacter vinelandii OPNA is a potential producer of large quantities of PHB, having ultra-high molecular weight by fed-batch production strategies. RESULTS: In batch and fed-batch cultures at different carbon–nitrogen molar ratios (10, 14 and 18), the OPNA strain produced PHB of high and ultra-high weight average molecular weight, with values between 2.3 and 6.6 MDa. In this strain, the weight average molecular weight was highly dependent on the initial carbon–nitrogen ratio, reaching the highest value (6.6 MDa) in batch cultures conducted with a carbon–nitrogen ratio of 18 and the lowest (2.3) with a carbon–nitrogen ratio of 10. On the other hand, using a two feeding-pulses fed-batch strategy, the highest global PHB volumetric productivity (0.56 g L−1 h−1) and polymer concentration (27.6 g L−1) were obtained. CONCLUSION: The OPNA strain accumulated up to 80% of PHB of ultra-high molecular weight as a function of the initial carbon–nitrogen molar ratio. Using a simple fed-batch strategy of two feeding-pulses was possible to produce up to 27 g L−1 of PHB.