Nitrogenase-catalyzed nitrogen fixation is a biochemically and genetically complex process requiring the participation of a number of different nif (nitrogen fixation) gene products. The nifF (electron transport), nifA (nif gene regulation) and nifE (FeMo-cofactor biosynthesis) genes from Azotobacter vinelandii were genetically analyzed. The nucleotide sequence of the nifF gene, which encodes a flavodoxin, was determined. Specific mutation strains indicated that in A vinelandii flavodoxin is not the unique physiological electron donor to nitrogenase. The nifF gene appears to be constitutively expressed but under nitrogen fixing conditions nifF gene expression is stimulated.