The complete amino acid sequence of the 7Fe ferredoxin from Azotobacter vinelandii (Av Fd) was determined by repetitive Edman degradation of the whole protein and of peptides derived from CNBr cleavage or chymotrypsin digestion. The sequence was confirmed by the 2A electron density maps for the residues calculated with difference Fourier coefficients. The density maps for all residues are included in the paper. Av Fd has several important differences with the clostridial-type ferredoxins: (i) Av Fd is 106 residues (versus 55-60 for other bacterial ferredoxins); (ii) Av Fd has 9 cysteines, one of which (residue 24) is not homologous with the bacterial ferredoxins; (iii) Av Fd has 2 extra residues between 2 cysteines (residues 11 and 16) homologous to cysteines in the bacterial ferredoxins; and (iv) Av Fd has the unique sequence -Cys-Val-Glu-Val-Cys- (residues 16-20) which are two of the ligands of the 3Fe:3S center. These sequence features are compared to the sequences of various ferredoxin groups. Structure predictions for other suspected 7Fe ferredoxins are discussed.