Azotobacter vinelandii strain ATCC 12837 and A. chroococcum strain H23 (CECT 4435) were able to grow on N-free or NH4Cl-amended chemically-defined (Burk's) media, with protocatechuic acid (1-2 mmol l(-1)) or sodium p-hydroxybenzoate (1-10 mmol l(-1)) as sole carbon (C) sources. At a concentration of 2 mmol l(-1), both substrates supported nitrogen fixation (acetylene reduction assay) at similar or higher rates than bacteria grown in control media amended with 2 mmol l(-1) sodium succinate as C source. The two strains produced the B-group vitamins niacin, pantothenic acid, thiamine, riboflavin and biotin after 72 h of growth in chemically-defined media with 2 mmol l(-1) protocatechuic acid, sodium p-hydroxybenzoate or sodium succinate as sole C source, either in N-free media or in media amended with 0.1% NH4Cl. Quantitative production of all vitamins was affected by the use of the different C and N substrates.