Alginate production by Azotobacter vinelandii was studied in batch and continuous cultures under microaerobic conditions. In batch culture at a pO(2) of 2-3% (air saturation) alginate production was enhanced by decreasing the PO43- level in the medium. Alginate yield from biomass (Y-P/X) reached the highest value of 0.66 g/g at the lowest phosphate level (100 mg/l), compared to 0.40 g/g and 0.25 g/g at higher phosphate levels (200 mg/l and 400 mg/l, respectively). In contrast, biomass formation behaved differently and the growth yield (Y-X/S) decreased with decreasing PO43- concentrations. Moreover, the respiratory quotient (RQ) of the culture was dependent on the initial phosphate concentration, especially in the phosphate-limited phase of growth. As the initial phosphate level decreased from 400 mg/l to 100 mg/l, the average RQ value of the culture declined from 1.46 to 0.89. The low RQ value is very close to the theoretical optimum RQ, calculated to be 0.8 on the basis of the stoichiometry of the metabolic pathways for alginate formation from sucrose. This optimum RQ was also confirmed in continuous culture at different dilution rates. Independent of the dilution rate, a pO(2) value of 2-5% (air saturation) was found to be optimal for alginate production, the corresponding RQ values being 0.80-0.84. In addition, the molecular mass and composition of alginate were also found to be affected by both phosphate and oxygen concentrations. In conclusion, the RQ appears to be a useful parameter for optimum control of alginate production with this microorganism.